v4l2bufferpool: Wait before polling if queue is empty

v4l2bufferpool: Wait before polling if queue is empty

In kernel before 3.17, polling during queue underrun would unblock right
away and trigger POLLERR. As we are not handling POLLERR, we would endup
blocking in DQBUF call, which won't be unblocked correctly when going
to NULL state. A deadlock at start caused by locking error in libv4l2 was
also seen before this patch. Instead, we wait until the queue is no longer
empty before polling.

https://bugzilla.gnome.org/show_bug.cgi?id=731015