avformat/vividas: simplify, use av_rescale_q() instead

avformat/vividas: simplify, use av_rescale_q() instead

note it'll cause a small difference in accuracy for the pts, please see the testing result below:
 $ wget http://samples.ffmpeg.org/archive/all/unknown+unknown+unknown+unknown+5029_paramount_en_1250.viv
 $ ./ffmpeg -t 0.04 -i ./unknown+unknown+unknown+unknown+5029_paramount_en_1250.viv -f null -
 old:
 pts: 522
 pts: 1044
 pts: 1567
 pts: 3918
 pts: 8097
 pts: 12277
 pts: 16457
 ...

 new:
 pts: 522
 pts: 1045
 pts: 1567
 pts: 3918
 pts: 8098
 pts: 12278
 pts: 16457
 ...

Signed-off-by: Limin Wang <lance.lmwang@gmail.com>