Nevermind, I found it. Had to add a second line to /etc/pm/ config.d/99local. It now contains the following:
DISPLAY_QUIRK_S3_MODE="true" QUIRKOPTS="true"
Whick quirk I pick doesn't make a difference, I found several that work. This one works just fine and the laptop goes to sleep when I enter start pm-suspend from a shell, close the lid, or select suspend from the klaptop menu.
Martin