surfaceflinger: give hwcomposer a chance to release buffers

Change-Id: I605fa779702022865dd58df3b36f37c2644ade36
diff --git a/services/surfaceflinger/DisplayHardware/HWComposer.cpp b/services/surfaceflinger/DisplayHardware/HWComposer.cpp
index 0291d78..129be4e 100644
--- a/services/surfaceflinger/DisplayHardware/HWComposer.cpp
+++ b/services/surfaceflinger/DisplayHardware/HWComposer.cpp
@@ -87,6 +87,11 @@
     return (status_t)err;
 }
 
+status_t HWComposer::release() const {
+    int err = mHwc->set(mHwc, NULL, NULL, NULL);
+    return (status_t)err;
+}
+
 size_t HWComposer::getNumLayers() const {
     return mList ? mList->numHwLayers : 0;
 }