media: atomisp: Split subdev and video-node registration into 2 steps

Split subdev and video-node registration into 2 steps, this is
a preparation step for moving video-node registration to the
end of probe() so that the loading() mutex can be removed.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>