vah264dec: picture rolls back to previous frame sometimes when decoding

All test file have been uploaded to here

Reproduce:

1. Env: Ubuntu:20.04, libva:2.8.0, gmmlib:intel-gmmlib-20.3.2, media-driver:intel-media-20.2.0, GStreamer:1.18
2. gst-launch-1.0 filesrc location=sample.h264 ! h264parse ! vah264dec ! vaapijpegenc ! multifilesink location="va/%04d.jpeg"
3. 2723.jpeg -> 2724.jpeg: stopwatch rolls back from 10:26.30 to 10:25.64
4. 2727.jpeg -> 2728.jpeg: stopwatch recovers from 10:25:44 to 10:26.49
5. change vah264dec to vaapih264dec: gst-launch-1.0 filesrc location=sample.h264 ! h264parse ! vaapih264dec ! vaapijpegenc ! multifilesink location="vaapi/%04d.jpeg", 2723.jpeg ~ 2728.jpeg: stopwatch displays fine