vaapidecode: simplify codec lookup from caps.
vaapidecode: simplify codec lookup from caps.

Add new gst_vaapi_codec_from_caps() helper to determine codec type from
the specified caps. Don't globally expose this function since this is
really trivial and only used in the vaapidecode element.

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