media: media/test-drivers: use vb2_video_unregister_device()

Use vb2_video_unregister_device() to automatically stop streaming
at unregister time.

This avoids the use of vb2_queue_release() which should not be
called by drivers that set vdev->queue.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>