v4l2: dequeue buffers properly on VIDIOC_STREAMOFF

v4l2: dequeue buffers properly on VIDIOC_STREAMOFF

<UPSTREAM MERGE NOT REQUIRED>

Instead of just marking all buffers as dequeued on STREAMOFF, properly
dequeue them -- this will, most notably, make sure that any mapped
DMABUF buffers will be properly unmapped.

BUG=chromium:232121
TEST=local build, run on CrOS snow

Change-Id: Ide111378caa70cf603b3edec65b4dedd0f1e1ea0
Signed-off-by: John Sheu <sheu@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/48346
Reviewed-by: Pawel Osciak <posciak@chromium.org>