media: imx-mipi-csis: Don't take lock in runtime PM handlers

The runtime PM handlers don't need manual locking as

- they are serialized by the runtime PM core
- they can't race with other functions taking the same lock, as they
  don't access any data protect by that lock

Drop the locking.

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>