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>
(cherry picked from commit 0e4bd71d)
3 jobs for modesetting_backports in 6 minutes and 47 seconds
Status Job ID Name Coverage
  Docker Image
passed #8832154
debian-testing

00:00:20

 
  Build And Test
passed #8832155
autotools-build-and-test

00:06:27

passed #8832156
meson-build-and-test

00:00:59