drm/exynos: Let core take care of normalizing the zpos

Instead of re-implementing the drm_atomic_helper_check() locally with just
adding drm_atomic_normalize_zpos() into it, set the
drm_mode_config->normalize_zpos.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
CC: Inki Dae <inki.dae@samsung.com>
CC: Joonyoung Shim <jy0922.shim@samsung.com>
CC: Seung-Woo Kim <sw0312.kim@samsung.com>
CC: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180321102029.15248-3-peter.ujfalusi@ti.com