media: atomisp: Make MC link from ISP to /dev/video# output node immutable

The link from the ISP's source pad to the /dev/video# output v4l2-dev
is always enabled and immutable, mark it as such.

Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>