media: rcar-vin: remove unneeded locking in async callbacks

The locking implemented in the async notifier callbacks are unnecessary
as the global list_lock in v4l2-async.c is held whenever one of the
callbacks are called.

The locking in itself is not harmful however it produces a LOCKDEP
warning between the global v4l2-async list_lock and the rcar-vin local
locking schema. Remove the rcar-vin locking for the async callbacks to
reduce complexity and silent the false LOCKDEP warning.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>