encoders: demote to RANK_NONE since not fit for autoplugging yet
encoders: demote to RANK_NONE since not fit for autoplugging yet

Encoders claim to support a whole bunch of input formats but then
just error out if the format is not actually supported, even if
there's a converter in front. This means they're not fit for
autoplugging in encodebin or camerabin yet and therefore should
not have a rank. People can still use them in custom pipelines.

https://bugzilla.gnome.org/show_bug.cgi?id=769266

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