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>