radeonsi: clean prefer_mono for the blit VS

This was missed when I was reworking shader key updates.

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13048>

src/gallium/drivers/radeonsi/si_state_shaders.c

@@ -1814,6 +1814,7 @@ void si_vs_key_update_inputs(struct si_context *sctx)
 
    if (vs->info.base.vs.blit_sgprs_amd) {
       si_clear_vs_key_inputs(sctx, key, &key->part.vs.prolog);
+      key->opt.prefer_mono = 0;
       return;
    }