drm: rcar-du: Handle event when disabling CRTCs

The driver currently handles vblank events only when updating planes on
a CRTC. The atomic update API however allows requesting an event when
disabling a CRTC. This currently leads to event objects being leaked in
the kernel and to events not being sent out. Fix it.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>