Set the drm_mode_config->normalize_zpos and call the generic
drm_atomic_helper_check() instead of duplicating it within
tegra_atomic_check().

Call tegra_display_hub_atomic_check() after the drm_atomic_helpre_check()
returned without error.
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
CC: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180321102029.15248-4-peter.ujfalusi@ti.com