Comment # 19 on bug 798308 from Egbert Eich

Right, you only have 2 crtcs but 3 outputs. Sharing CRTCs for any of these
won't work, therefore you need to shut off one of the outputs.
Still, from your tests I gather that DisplayPort-0 comes on but doesn't display
anything.

This seems to be a persistent problem - persistent across several versions -
which doesn't get resolved by 'accident'.
It would be instructive to see if another display with a different cable will
light up.
Not sure where to go from here...