avcodec: va: fix VAAPI disabled after a seek or format update on newest ffmpeg

This MR fixes the following error when seeking with VAAPI:

[h264 @ 0x7f8d500e7580] A hardware frames or device context is required for hardware accelerated decoding.
[h264 @ 0x7f8d500e7580] Failed setup for format vaapi: hwaccel initialisation returned error.
[00007f8d5003a2b0] avcodec decoder error: existing hardware acceleration cannot be reused

This is an alternative to the va->restart callback from !6420 (merged) If finally happen with all codecs, not only AV1.