media: imx: imx7_mipi_csis: Minimize locking in get/set format

Reduce the code sections that are run with the lock held in the get/set
format handlers:

- mipi_csis_get_format() retrieves a pointer to the format, and thus
  doesn't need locking as long as the arguments passed to the function
  don't require locking either.

- sdformat is a structure passed by the caller, not an internal state,
  and thus doesn't require locking.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Tested-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>