[media] media: vb2: return for polling if a buffer is available

The vb2_poll() does not need to wait next vb_buffer_done() if there is already
a buffer in done_list of queue, but current vb2_poll() always waits.
So done_list is checked before calling poll_wait().
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/v4l2-core/videobuf2-core.c

@@ -2014,6 +2014,7 @@ unsigned int vb2_poll(struct vb2_queue *q, struct file *file, poll_table *wait)
 	if (list_empty(&q->queued_list))
 		return res | POLLERR;
 
+	if (list_empty(&q->done_list))
 		poll_wait(file, &q->done_wq, wait);
 
 	/*