drm: renesas: shmobile: Rename shmob_drm_plane.plane

Rename the "plane" member of the shmob_drm_plane subclass structure to
"base", to improve readability.

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/73809d0a94f9075dd868cf567790d10f8ae61603.1694767209.git.geert+renesas@glider.be