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)
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00:00:35

 
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passed autotools-build-and-test #10857208

00:04:43

passed meson-build-and-test #10857209

00:01:47