compositor-drm: Relax plane restrictions for atomic

Since we now incrementally test atomic state as we build it, we can
loosen restrictions on what we can do with planes, and let the kernel
tell us whether or not it's OK.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>