[media] media: vb2: reset queued list on REQBUFS(0) call

Queued list was not reset on REQBUFS(0) call. This caused to enqueue a
freed buffer to the driver.

Reported-by: Angela Wan <angela.j.wan@gmail.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>