drm/sun4i: Send vblank event when the CRTC is disabled

So far, we were missing to send the vblank event when disabling the CRTC,
making us never report the last vblank event.

This was causing a time out on the page flip, which should be solved now.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>