stream: allow buffers to be allocated on uvc side

We want to allow video sources whose data is generated in userspace.
Currently buffers are allocated on the V4L2-backed video source and
exported to the uvc-gadget's V4L2 device. Since a video source with
data generated in userspace will not be backed by a V4L2 device, we
allow a way for buffers to be allocated on uvc-gadget's V4L2 device.

As a corollary, the way the buffers are passed has to be changed as
well. Previously they were queued and dequeued between the V4L2 video
source and uvc and vice versa. To allow a video source not backed by
a V4L2 device, we allow an alternative flow where they are dequeued and
queued from and to uvc, and giving a chance to the video source to fill
the buffer in between.

Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

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