remove a dependency of DisplayDevice on the refresh rate

this remove a dependency (not all) on FramebufferSurface

Change-Id: Ie07ce70760cdcedfb41b5b41bea8da45637bf474
diff --git a/services/surfaceflinger/Layer.cpp b/services/surfaceflinger/Layer.cpp
index 7da0c9b..f72000f 100644
--- a/services/surfaceflinger/Layer.cpp
+++ b/services/surfaceflinger/Layer.cpp
@@ -729,7 +729,7 @@
     LayerBaseClient::dumpStats(result, buffer, SIZE);
     const size_t o = mFrameLatencyOffset;
     const DisplayDevice& hw(mFlinger->getDefaultDisplayDevice());
-    const nsecs_t period = hw.getRefreshPeriod();
+    const nsecs_t period = mFlinger->getHwComposer().getRefreshPeriod();
     result.appendFormat("%lld\n", period);
     for (size_t i=0 ; i<128 ; i++) {
         const size_t index = (o+i) % 128;