media: atomisp: Implement link_setup() op for ISP subdev MC entity

The atomisp driver's Android heritage makes it weird in that
even though it uses MC + subdev-s it is designed to primarily
be controlled through its /dev/video# node.

It implements s_input() on /dev/video# to select which sensor to use,
while ignoring link_setup() calls to enable a link to another sensor.

Add support for selecting the active sensor the MC way by adding
link_setup() support.

The implementation is a bit convoluted due to the atomisp driver's
heritage.

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>