drm: Add drm_mode_config->normalize_zpos boolean

Instead of drivers duplicating the drm_atomic_helper_check() code to be
able to normalize the zpos they can use the normalize_zpos flag to let the
drm core to do it.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180321102029.15248-2-peter.ujfalusi@ti.com