encoder: h264: generate new SPS only when codec config changed.

encoder: h264: generate new SPS only when codec config changed.

It is not necessary to periodically send SPS or subset SPS headers.
This is up to the upper layer (e.g. transport layer) to decide on
if/how to periodically submit those. For now, only generate new SPS
or subset SPS headers when the codec config changed.

Note: the upper layer could readily determine the config headers
(SPS/PPS) through the gst_vaapi_encoder_h264_get_codec_data() function.

https://bugzilla.gnome.org/show_bug.cgi?id=732083

Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>