media: imx: imx7_mipi_csis: Move link setup check out of locked section

Checking if the link setup operation is called for the sink or source
pad doesn't require any locking. Move it out of the section protected by
the mutex.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>