plugins: encode: vp9: Implement vp9's allowed_profiles() func.
plugins: encode: vp9: Implement vp9's allowed_profiles() func.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-vaapi/-/merge_requests/188>

ceyusa/gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,GStreamer/gstreamer-vaapi