don't actuate vsync hint when not needed

Change-Id: I61c43dd76041816ab8cbe9aeaa55c11c1479ed03
diff --git a/services/surfaceflinger/DisplayHardware/PowerHAL.h b/services/surfaceflinger/DisplayHardware/PowerHAL.h
index 929bc03..ef67b8f 100644
--- a/services/surfaceflinger/DisplayHardware/PowerHAL.h
+++ b/services/surfaceflinger/DisplayHardware/PowerHAL.h
@@ -36,6 +36,7 @@
 
 private:
     power_module_t*   mPowerModule;
+    bool mVSyncHintEnabled;
 };
 
 // ---------------------------------------------------------------------------