drm/sti: atomic crtc/plane update

Better fit STI hardware structure.
Planes are no more responsible of updating mixer information such
as z-order and status. It is now up to the CRTC atomic flush to
do it. Plane actions (enable or disable) are performed atomically.
Disabling of a plane is synchronize with the vsync event.

Signed-off-by: Vincent Abriou <vincent.abriou@st.com>
Reviewed-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>