now able to set the layer stack on a DisplayDevice

Change-Id: Ia9691cf221b9444c243eb468d9e276a30e600b6b
diff --git a/services/surfaceflinger/DisplayDevice.cpp b/services/surfaceflinger/DisplayDevice.cpp
index 56ac635..326a124 100644
--- a/services/surfaceflinger/DisplayDevice.cpp
+++ b/services/surfaceflinger/DisplayDevice.cpp
@@ -313,6 +313,13 @@
 
 // ----------------------------------------------------------------------------
 
+void DisplayDevice::setLayerStack(uint32_t stack) {
+    mLayerStack = stack;
+    dirtyRegion.set(bounds());
+}
+
+// ----------------------------------------------------------------------------
+
 status_t DisplayDevice::orientationToTransfrom(
         int orientation, int w, int h, Transform* tr)
 {