hwc: Add binder interface to get the visibleRegion

- This binder interface can be used by clients to know the
  active visible region for a display(pri/ext/virt)
- When external orientation is used, return the destFrame of
  the FrameBuffer layer, as its the viewFrame

Change-Id: I7cfd149c76c16b9a3031103c89b1932d44bcbecd
diff --git a/libhwcomposer/hwc_utils.h b/libhwcomposer/hwc_utils.h
index 872b306..cd84f73 100644
--- a/libhwcomposer/hwc_utils.h
+++ b/libhwcomposer/hwc_utils.h
@@ -87,6 +87,8 @@
     bool isConfiguring;
     // External Display is in MDP Downscale mode indicator
     bool mDownScaleMode;
+    // Ext dst Rect
+    hwc_rect_t mDstRect;
 };
 
 struct ListStats {