h264: set default values for some header fields.

h264: set default values for some header fields.

The SPS, PPS and slice headers are not fully zero-initialized in the
codecparsers/ library. Rather, the standard upstream behaviour is to
initialize only certain syntax elements with some inferred values if
they are not present in the bitstream.

At the gstreamer-vaapi decoder level, we need to further initialize
certain syntax elements with some sensible default values so that to
not complicate VA drivers that just pass those verbatim to the HW,
and also avoid an memset() of the whole decoder unit.

Signed-off-by: Sreerenj Balachandran <sreerenj.balachandran@intel.com>
Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>