drm: rcar-du: Fix leak of CMM platform device reference

The device references acquired by of_find_device_by_node() are not
released by the driver. Fix this by registering a cleanup action.

Fixes: 8de707ae ("drm: rcar-du: kms: Initialize CMM instances")
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>