drm/sti: make crct disable atomic

Wait until the next vblank to be sure that crtc has been disabled.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20181012094639.1585-1-benjamin.gaignard@linaro.org