media: rcar-isp: Convert to subdev state

Convert to subdev state. This allows us to drop the internal mutex and
risp_get_pad_format(), and add streams support in the future.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>