drm/sti: 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.

Note: the drm_atomic_helper_check() now includes

if (state->legacy_cursor_update)
	state->async_update = !drm_atomic_helper_async_check(drm, state);

which was added after the driver moved away from using it
(38d868e4 drm: Don't force all planes to
be added to the state due to zpos)

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
CC: Benjamin Gaignard <benjamin.gaignard@linaro.org>
CC: Vincent Abriou <vincent.abriou@st.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180321102029.15248-5-peter.ujfalusi@ti.com