compositor-drm: Introduce drm_pending_state structure

drm_pending_state is currently skeletal, but will be used to retain
data through begin_repaint -> assign_planes -> repaint -> repaint_flush.

The flush and cancel functions are currently identical, only freeing the
state, but they will be used for different purposes in later patches.
Specifically, the intent is to apply any pending output changes (through
PageFlip/SetCrtc, or the atomic ioctls) in flush, and only free the
state in cancel.

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