drm: rcar-du: Embed drm_device in rcar_du_device

Embedding drm_device in rcar_du_device allows usage of the DRM managed
API to allocate both structures in one go, simplifying error handling.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>