media: imx-mipi-csis: Implement .init_cfg() using .set_fmt()

The .set_fmt() handler is responsible for adjusting the requested format
based on the device limitations. Implement .init_cfg() as a wrapper of
.set_fmt(), to ensure that the initial configuration always matches the
rules implemented in .set_fmt(), should they ever change.

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>