drm/atomic: Make drm_atomic_plane_disabling easier to understand.

drm/atomic: Make drm_atomic_plane_disabling easier to understand.

This function becomes a lot simpler when having passed both the old and
new state to it. Looking at all callers, it seems that old_plane_state
is never NULL so the check can be dropped.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1487256430-7625-3-git-send-email-maarten.lankhorst@linux.intel.com
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>