media: imx: imx7-media-csi: 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 state stored in the
imx7_csi structure, but also by replacing the manual lock with the state
lock.

The is_streaming field is now protected by the active state lock, either
explicitly in .s_stream(), where the active state is locked manually, or
implicitly in .set_fmt(), which is called with the state locked.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Adam Ford <aford173@gmail.com> #imx8mm-beacon-kit
Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Tested-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>