media: imx: imx8mq-mipi-csi2: Use V4L2 subdev active state

Simplify the driver by using the V4L2 subdev active state API to store
the active format.

Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>