media: ipu3-cio2: Proceed with notifier init even if there are no subdevs

The notifier may be registered even if there are no subdevs. Do that to
simplify the code.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>