improve logging of external displays

Change-Id: I041aebb7fc655aeca98bbf698d15e05d7c12cac9
diff --git a/services/surfaceflinger/DisplayDevice.h b/services/surfaceflinger/DisplayDevice.h
index e3b8d57..c9d21e4 100644
--- a/services/surfaceflinger/DisplayDevice.h
+++ b/services/surfaceflinger/DisplayDevice.h
@@ -138,7 +138,7 @@
      * Debugging
      */
     uint32_t getPageFlipCount() const;
-    void dump(String8& res) const;
+    void dump(String8& result, char* buffer, size_t SIZE) const;
 
 private:
     void init(EGLConfig config);