media: imx: imx7_mipi_csis: Don't take state->lock in .link_setup()

The .link_setup() handler uses state->lock to protect the src_sd field.
This is only used in mipi_csis_s_stream(), which can't race
.link_setup() as the MC core prevents link setup when the pipeline is
streaming. Drop the lock.

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