drm: sti: use drm_crtc_vblank_{on/off} instead of drm_vblank_{on/off}

Make sure that vblank is enabled when crtc commit is call.
Replace drm_vblank_off() by drm_crtc_vblank_off()

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>