drm: rcar-du: Don't put reference to drm_device in rcar_du_remove()

The reference to the drm_device that was acquired by
devm_drm_dev_alloc() is released automatically by the devres
infrastructure. It must not be released manually, as that causes a
reference underflow..

Fixes: ea6aae15

 ("drm: rcar-du: Embed drm_device in rcar_du_device")
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>