DisplayDevice doesn't need weak pointers or polymorphism

let's use a LightRefBase for now.

Change-Id: Ida068fa902343a8f89ffb6d34c2cd4627ab43c6c
diff --git a/services/surfaceflinger/DisplayDevice.h b/services/surfaceflinger/DisplayDevice.h
index 9bcb6fd..b34bcf1 100644
--- a/services/surfaceflinger/DisplayDevice.h
+++ b/services/surfaceflinger/DisplayDevice.h
@@ -39,7 +39,7 @@
 class LayerBase;
 class SurfaceFlinger;
 
-class DisplayDevice : public virtual RefBase
+class DisplayDevice : public LightRefBase<DisplayDevice>
 {
 public:
     // region in layer-stack space
@@ -65,7 +65,7 @@
             const sp<ANativeWindow>& surface,
             EGLConfig config);
 
-    virtual ~DisplayDevice();
+    ~DisplayDevice();
 
     // whether this is a valid object. An invalid DisplayDevice is returned
     // when an non existing id is requested