rtph265pay: Clean up _payload_nal
rtph265pay: Clean up _payload_nal

Move determining whether we need to fragment at all into the
fragmenter.

Align with rtph264pay

pexip/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good