Add a zpos property to planes on the DP crtc. Instead of calling
drm_atomic_helper_check_modeset() and drm_atomic_helper_check_planes()
manually, call drm_atomic_helper_check(), to also normalizes zpos in
plane state. Always update plane order and alpha settings during plane
commit, to allow changing the zpos property without modeset.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>