media: rcar-vin: Do not hold the group lock when unregistering notifier

There is no need to hold the group lock when unregistering the notifier,
but doing so triggers a warning about a possible cyclic lock dependency.

The lock warning cover the case where a subdevice is about to be bound
to the notifier at the same time as it's unregistered. The locking for
this scenario is handled in the v4l2-async framework so it's safe to
remove the lock in the driver. This match the locking logic in the
driver for when the notifier in question is registered.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>