encoder: h264: submit sequence parameter only once.
encoder: h264: submit sequence parameter only once.

Submit sequence parameter buffers only once, or when the bitstream
was reconfigured in a way that requires such. Always submit packed
sequence parameter buffers at I-frame period, if the VA driver needs
those.

https://bugzilla.gnome.org/show_bug.cgi?id=722737

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