media: imx: imx7-media-csi: Drop unneeded src_sd check

The .s_stream() and .link_validate() operations can't be called with a
NULL src_sd, as subdev nodes are not registered before the async
notifier completes. Remove the unneeded checks.

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>