frontends/va: Parse enc_constraint_set_flags from packed SPS

Reviewed-by: Ruijing Dong <ruijing.dong@amd.com>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>

(Reviewed in PR !26556 as well)

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26573>