mpeg2: fix is_first_field calculation.
mpeg2: fix is_first_field calculation.

Reset is_first_field for frame pictures. Factor out locations where
the flag is updated.

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