media: rcar-vin: Improve reuse of parallel notifier

In preparation for adding a new media graph layout move the code reuse
of the parallel notifier setup from probe directly to the current media
graph initialization function. This is needed as there will be no
parallel interface in the new graph layout.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>