drm/atomic: Add sanity checks to drm_atomic_helper_async_commit()

->atomic_async_update() requires that drivers update the plane->state
object before returning. Make sure at least common properties have been
updated.

Cc: Gustavo Padovan <gustavo@padovan.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Acked-by: Eric Anholt <eric@anholt.net>
Acked-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180330145518.29770-1-boris.brezillon@bootlin.com