[CentOS] External monitor support changes.

I've been working on this issue for quite a while.

Back in January of 2015 I posted about how well external monitors worked 
with the out-of-the-box CentOS 7 ATI radeon drivers and my Dell M6500 
with an ATI FirePro M7820.

At the time the support was seamless and 'Just Worked' without any 
issue.  A few months later, some update came down the pipe and this 
broke completely, with the same projector that had been working fine for 
a while. It just simply quit working.

The docked double monitors still worked fine, and it wasn't a critical 
issue for me, so I kept trying it occasionally to see if it would start 
working again, but it never did.

So last week I upgraded laptops to a Dell Precision M6700 with an nVidia 
card, upon which I immediately installed the ELrepo drivers. The docked 
dual monitors worked fine (used in a triple monitor setup, just like the 
ATI card in the M6500).  So I decided I would try the HDMI port.  Well, 
it did exactly the same thing the M6500 with the ATI card did.

The symptom is that the laptop screen will blank, then come back for a 
bit, then blank for a few seconds then come back; lather, rinse, 
repeat.  The Xorg log shows this:
+++++++++++++++++++++++++
[   239.338] (**) NVIDIA(0): Using HorizSync/VertRefresh ranges from the 
EDID for display
[   239.338] (**) NVIDIA(0):     device DELL 7700 (DFP-1) (Using EDID 
frequencies has been
[   239.338] (**) NVIDIA(0):     enabled on all display devices.)
[   239.344] (--) NVIDIA(GPU-0): DELL 7700 (DFP-1): connected
[   239.344] (--) NVIDIA(GPU-0): DELL 7700 (DFP-1): Internal TMDS
[   239.344] (--) NVIDIA(GPU-0): DELL 7700 (DFP-1): 165.0 MHz maximum 
pixel clock
[   239.344] (--) NVIDIA(GPU-0):
[   239.459] (II) NVIDIA(0): Setting mode "DP-0: nvidia-auto-select 
@1920x1080 +1920+0 {ViewPortIn=1920x1080, ViewPortOut=1920x1080+0+0}"
[   240.048] (II) NVIDIA(0): Setting mode "LVDS-0: nvidia-auto-select 
@1920x1080 +0+0 {ViewPortIn=1920x1080, ViewPortOut=1920x1080+0+0}, DP-0: 
nvidia-auto-select @1920x1080 +1920+0 {ViewPortIn=1920x1080, 
ViewPortOut=1920x1080+0+0}"
[   240.542] (--) NVIDIA(GPU-0): CRT-0: disconnected
[   240.542] (--) NVIDIA(GPU-0): CRT-0: 400.0 MHz maximum pixel clock
[   240.542] (--) NVIDIA(GPU-0):
[   240.542] (--) NVIDIA(GPU-0): CMN (DFP-0): connected
[   240.542] (--) NVIDIA(GPU-0): CMN (DFP-0): Internal LVDS
[   240.542] (--) NVIDIA(GPU-0): CMN (DFP-0): 330.0 MHz maximum pixel clock
[   240.542] (--) NVIDIA(GPU-0):
[   240.566] (--) NVIDIA(GPU-0): DELL 7700 (DFP-1): connected
[   240.566] (--) NVIDIA(GPU-0): DELL 7700 (DFP-1): Internal TMDS
[   240.566] (--) NVIDIA(GPU-0): DELL 7700 (DFP-1): 165.0 MHz maximum 
pixel clock
+++++++++++++++++++++++

The nVidia control panel shows the DELL 7700 coming up, then going away, 
then coming up, then going away.  I hadn't tried the M6500 in a while 
with it, so I'm going to try it again and look at the Xorg log, but my 
recollection is that the symptoms and log were the same.  There isn't 
any error condition reported in the Xorg log; it just doesn't ever 
display anything on the projector.

While I realize that the nvidia drivers are ELrepo packages, the same 
symptoms happened with the M6500's ATI with the stock CentOS 7 ATI 
drivers; it worked beautifully one day, then some updates came, and it 
just quit working, using the same projector, cables, and receiver.  I 
don't recall exactly where in the 7.x cycle this happened, 
unfortunately.  I may give the original 7.0 a whirl then the original 
7.1 a whirl, but I think it started with the 7.2 release.  It could be 
the receiver in the middle, and I'll try a direct cable to the 
projector.  But I tried that with the M6500 and it didn't change anything.

But, again, the docked triple-head configuration works like a champ.

I will perform more testing and update as I find things.