better fix for [2420565] Surface.lockCanvas() updates the dirty region too often

Change-Id: I83438b40effd21538f1c74396dc665254b9d5ab6
diff --git a/include/surfaceflinger/Surface.h b/include/surfaceflinger/Surface.h
index 68809a9..40dc467 100644
--- a/include/surfaceflinger/Surface.h
+++ b/include/surfaceflinger/Surface.h
@@ -250,7 +250,7 @@
     sp<GraphicBuffer>           mLockedBuffer;
     sp<GraphicBuffer>           mPostedBuffer;
     mutable Region              mOldDirtyRegion;
-    bool                        mNeedFullUpdate;
+    bool                        mReserved;
 
     // query() must be called from dequeueBuffer() thread
     uint32_t                    mWidth;