drm: rcar-du: Let core take care of normalizing the zpos

Set the drm_mode_config->normalize_zpos and call drm_atomic_helper_check()
from rcar_du_atomic_check() instead of re implementing the function locally.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
CC: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180321102029.15248-6-peter.ujfalusi@ti.com