demux: fix video with demux_mpg (DVD playback)

Commit 4d016a9 changed how demuxers report the codec of each stream.
Some of that was missed in video.c, which is normally not used, except
for DVD playback (for which the legacy demux_mpg demuxer is used).

Not sure about the ASF/AVI related change, but these also affect
playback with legacy demuxers only.