iris: Set 3DSTATE_WM::ForceThreadDispatchEnable

The Vulkan driver only sets this if color writes are disabled, which
is more conservative - but would require us to inspect blend state.

(If color writes are enabled, we don't need to force anything, because
the internal signal is already correct.  But it shouldn't hurt to do so.)

src/gallium/drivers/iris/iris_state.c

@@ -4407,6 +4407,10 @@ iris_upload_dirty_render_state(struct iris_context *ice,
             wm.EarlyDepthStencilControl = EDSC_PREPS;
          else if (wm_prog_data->has_side_effects)
             wm.EarlyDepthStencilControl = EDSC_PSEXEC;
+
+         /* We could skip this bit if color writes are enabled. */
+         if (wm_prog_data->has_side_effects || wm_prog_data->uses_kill)
+            wm.ForceThreadDispatchEnable = ForceON;
       }
       iris_emit_merge(batch, cso->wm, dynamic_wm, ARRAY_SIZE(cso->wm));
    }