vaapidecode: always a valid format in decoded_info
vaapidecode: always a valid format in decoded_info

Always set a valid format in decoded_info class variable.

https://bugzilla.gnome.org/show_bug.cgi?id=764316

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