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>
3 jobs for fix_possible_crtcs in 9 minutes and 21 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Docker Image
passed #839212
debian-testing

00:02:03

 
  Build And Test
passed #839213
autotools-build-and-test

00:07:18

passed #839214
meson-build-and-test

00:02:26