drm: rcar-du: Wait for page flip completion when turning the CRTC off

Turning a CRTC off will prevent a queued page flip from ever completing,
potentially confusing userspace. Wait for queued page flips to complete
before turning the CRTC off to avoid this.

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