decoder: h264: decode current picture earlier.
decoder: h264: decode current picture earlier.

Slightly optimize decoding process by submitting the current VA surface
for decoding earlier to the hardware, and perform the reference picture
marking process and DPB update process afterwards.

This is a minor optimization to let the video decode engine kick in work
earlier, thus improving parallel resources utilization.

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