surfaceflinger: give hwcomposer a chance to release buffers

Change-Id: I605fa779702022865dd58df3b36f37c2644ade36
diff --git a/services/surfaceflinger/DisplayHardware/HWComposer.h b/services/surfaceflinger/DisplayHardware/HWComposer.h
index c5d5c2b..22ff10c 100644
--- a/services/surfaceflinger/DisplayHardware/HWComposer.h
+++ b/services/surfaceflinger/DisplayHardware/HWComposer.h
@@ -48,6 +48,8 @@
     // commits the list
     status_t commit() const;
 
+    // release hardware resources
+    status_t release() const;
 
     size_t getNumLayers() const;
     hwc_layer_t* getLayers() const;