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

drivers/gpu/drm/sti/sti_crtc.c

@@ -40,6 +40,8 @@ static void sti_crtc_atomic_disable(struct drm_crtc *crtc,
 	DRM_DEBUG_DRIVER("\n");
 
 	mixer->status = STI_MIXER_DISABLING;
+
+	drm_crtc_wait_one_vblank(crtc);
 }
 
 static int