modesetting: Fix possible_crtcs

Populate outout possible_crtcs as the union of possible_crtcs from the encoders rather than the intersection. Otherwise we're easily left with possible_crtcs==0 when all the possible encoders have non-overlapping possible_crtcs.

No idea what the magic 0x7f is about, but keep it around in case it matters.

Signed-off-by: Ville Syrjälä ville.syrjala@linux.intel.com