media: imx-mipi-csis: Use V4L2 subdev active state

Use the V4L2 subdev active state API to store the active format. This
simplifies the driver not only by dropping the mipi_csis_device csis_fmt
and format_mbus fields, but it also allows dropping the device lock,
replaced with the state lock.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Adam Ford <aford173@gmail.com> #imx8mn-beacon
Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>