Turn the custom formats initialization function into a standard
pad::init_cfg handler and use it in subdevs instead of initializing
formats in the subdev open handler.

This makes the subdev open handler empty, so remove it.
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>