drm: renesas: shmobile: Embed drm_device in shmob_drm_device

Embedding drm_device in shmob_drm_device allows us to use the DRM
managed API to allocate both structures in one go, simplifying error
handling.

Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/abbf95021191824f44cc8c52e4afbde93d44363a.1694767209.git.geert+renesas@glider.be