mpeg2: fix slice_vertical_position calculation (again).
mpeg2: fix slice_vertical_position calculation (again).

VA-API expects slice_vertical_position as the initial position from the
bitstream. i.e. the direct slice() information. VA drivers will be fixed
accordingly.

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