st/nine: Fix incompatible-pointer-types-discards-qualifiers errors.

st/nine: Fix incompatible-pointer-types-discards-qualifiers errors.

../src/gallium/state_trackers/nine/nine_ff.c:129:28: error: initializing 'struct nine_ff_vs_key *' with an expression of type 'const void *' discards qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
    struct nine_ff_vs_key *vs = key;
                           ^    ~~~
../src/gallium/state_trackers/nine/nine_ff.c:145:28: error: initializing 'struct nine_ff_ps_key *' with an expression of type 'const void *' discards qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
    struct nine_ff_ps_key *ps = key;
                           ^    ~~~

Fixes: fdd96578ef2d ("nine: Add state tracker nine for Direct3D9 (v3)")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Andre Heider <a.heider@gmail.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4015>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4015>