drm/vc4: Send a VBLANK event when disabling a CRTC

VBLANK events are missed when the CRTC is being disabled because the
driver does not wait till the end of the frame before stopping the
HVS and PV blocks. In this case, we should explicitly issue a VBLANK
event if there's one waiting.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/1497601833-24588-1-git-send-email-boris.brezillon@free-electrons.com