drm/sti: Use drm_crtc_vblank_*() API

Non-legacy drivers should only use this API to allow per-CRTC data to be
eventually moved into struct drm_crtc.

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