radeonsi/vcn: Improve DPB handling with missing references

Currently when there are missing references, DPB will contain empty slots in place of the missing references and may also put the valid references at incorrect indices. This further increases the error from decoding corrupted frames and even causes incorrect output when a frame should be decoded correctly (when it doesn't actually use the missing references) otherwise.

Also include H264 decoding workaround to fix #11060 (closed)