vaapih264enc: update level in src caps
vaapih264enc: update level in src caps

And, if downstream requests a specific level, the caps are not
negotiated, because there is no mechanism right now to specify a
custom level in the internal encoder.

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