d3d12: VP9 Decode - Fix use_prev_in_find_mvs_refs calculation

Fixes: c8e8ce83 ("d3d12: Add VP9 Decode support")

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21694>
(cherry picked from commit fb82371558a5f8c0293079e2de292e7723c90915)
2 files changed