media: staging/imx: Implement init_cfg subdev pad op

Implement the init_cfg pad op in all imx-media subdevices. The try
formats are initialized to the current active formats on all pads.

Signed-off-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>