compositor-drm: Ignore occluded views

When trying to assign planes, keep track of the areas which are
already occluded, and ignore views which are completely occluded. This
allows us to build a state using planes only, when there are occluded
views which cannot go into a plane behind views which can.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Tested-by: Emre Ucan <eucan@de.adit-jv.com>