decoder: h264: factor out DPB pruning for MVC.
decoder: h264: factor out DPB pruning for MVC.

Factor out the removal process of unused inter-view only reference
pictures from the DPB, prior to the possible insertion of the current
picture.

Ideally, the compiler could still opt for generating two loops. But
at least, the code is now clearer for maintenance.

CapOM/gstreamer-vaapi,GStreamer/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,CapOM/gstreamer-vaapi,CapOM/gstreamer-vaapi,ceyusa/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi