compositor-drm: Return plane state from plane preparation

Return a pointer to the plane state, rather than returning its
underlying weston_plane. This eliminates any ambiguity between placing
client buffers on planes, and placing them through the renderer.

drm_output_propose_state is only concerned with preparing, testing, and
returning DRM state objects. Assigning views to weston_planes only
happens later, inside drm_assign_planes. This makes that split more
clear.

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