media: adv748x-csi2: Use the subdev active state

Initialize and use the subdev active state to store the subdevice
format.

This simplifies the implementation of the get_fmt and set_fmt pad
operations.

Signed-off-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Tested-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20240617161135.130719-9-jacopo.mondi@ideasonboard.com


Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>