amfencoder: always empty the queue when pushing output samples

amfencoder: always empty the queue when pushing output samples

gst_amf_encoder_try_output() pushes at most one output buffer downstream
although more may be ready. As a consequence, output samples will keep
queueing up in AMFComponent whenever QueryOutput() returns AMF_REPEAT
(and do_wait is FALSE). This has negative impact on latency when the
video being encoded is a live stream.

In order to avoid it, always retrieve and push all samples available in
AMFComponent's output queue at once.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2536>