drm: renesas: shmobile: Replace .dev_private with container_of()

Now that drm_device is embedded in shmob_drm_device, we can use
a container_of()-based helper to get the shmob_drm_device pointer from
the drm_device, instead of using the deprecated drm_device.dev_private
field.

While at it, restore reverse Xmas tree ordering of local variable
declarations.

Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Sui Jingfeng <suijingfeng@loongson.cn>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/8ef4be8bffe75efc7f4b66f3732ec357f7d43e0f.1694767209.git.geert+renesas@glider.be