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.