drm/sti: in crtc_atomic_flush, enable only planes of this crtc

crtc_atomic_flush performs some additional processing, like plane
enable at mixer level.
Enable only the planes attached to the CRTC.

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Acked-by: Vincent Abriou <vincent.abriou@st.com>