[media] pwc: Fix locking

My last locking rework for pwc mistakenly assumed that videbuf2 does its
own locking, but it does not! This patch fixes the missing locking by
moving over the the video_device lock, and introducing a separate lock
for the videobuf2_queue.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>