llvmpipe: disable opaque variant for multisample

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4122>

src/gallium/drivers/llvmpipe/lp_state_fs.c

@@ -3210,6 +3210,7 @@ generate_variant(struct llvmpipe_context *lp,
          fullcolormask &&
          !key->stencil[0].enabled &&
          !key->alpha.enabled &&
+         !key->multisample &&
          !key->blend.alpha_to_coverage &&
          !key->depth.enabled &&
          !shader->info.base.uses_kill &&