drm: rcar-du: Put reference to VSP device

The reference to the VSP device acquired with of_find_device_by_node()
in rcar_du_vsp_init() is never released. Fix it with a drmm action,
which gets run both in the probe error path and in the remove path.

Fixes: 6d62ef3a ("drm: rcar-du: Expose the VSP1 compositor through KMS planes")
Reported-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>