dav1d: update to 1.2.1.

1.2.1 is a small release of dav1d, adding more SIMD and fixes

- Fix a threading race on task_thread.init_done
- NEON z2 8bpc and high bit-depth optimizations
- SSSE3 z2 high bit-depth optimziations
- Fix a desynced luma/chroma planes issue with Film Grain
- Reduce memory consumption
- Improve dav1d_parse_sequence_header() speed
- OBU: Improve header parsing and fix potential overflows
- OBU: Improve ITU-T T.35 parsing speed
- Misc buildsystems, CI and headers fixes

(wiz)