compositor-drm: Split drm_assign_planes in two

Move drm_assign_planes into two functions: one which proposes a plane
configuration, and another which applies that state to the Weston
internal structures. This will be used to try multiple configurations
and see which is supported.

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>