backend-drm: Use aspect-ratio bit definitions from libdrm

backend-drm: Use aspect-ratio bit definitions from libdrm

When the aspect-ratio-aware mode support was added to Weston, it was
done before the libdrm support was finalised and merged. Between it
being added to Weston and being merged, it changed to no longer provide
the offset for the bitmask.

Instead of using the mask and a compatible enum, if we update our
libdrm dependency, we can use the flag definitions directly from libdrm.

In 94e4068ba171, the libdrm dependency was bumped to 2.4.83, which
enabled us to remove a bunch of error-prone ifdefs by making atomic and
modifier support mandatory.

We determined in the discussion of !311 that it was safe to push the
dependency as high as 2.4.91, as that was what was available in major
distributions.

Bumping to 2.4.86 allows us to safely remove the ifdef and go with
upstream flags, as that was added in mesa/drm@0d889201d106.

Signed-off-by: Daniel Stone <daniels@collabora.com>