[media] pwc: Use vb2 queue mutex through a single name

This lock was being taken using two different names
(pointers) in the same function.
Both names refer to the same lock,
so this wasn't an error; but it looked very strange.

Cc: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Ezequiel Garcia <elezegarcia@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>