[media] vb2: fix vb2_poll for output streams

vb2_poll should always return POLLOUT | POLLWRNORM as long as there
are fewer buffers queued than there are buffers available. Poll for
an output stream should only wait if all buffers are queued and nobody
is dequeuing them.

Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>