get rid off preserve backbuffer optimization in SF

this optimization didn't improve performance and in fact
seemed to hurt more than anything else. it also made
things a lot more complex as it introduced edges cases
when switching to/from h/w composer.

Change-Id: Iaafc235e175f5740cd98bff914d706e02ab88bb8
diff --git a/services/surfaceflinger/SurfaceFlinger.h b/services/surfaceflinger/SurfaceFlinger.h
index 46bbab2..1aea452 100644
--- a/services/surfaceflinger/SurfaceFlinger.h
+++ b/services/surfaceflinger/SurfaceFlinger.h
@@ -318,7 +318,7 @@
             void        handleWorkList();
             void        handleRepaint();
             void        postFramebuffer();
-            void        setupHardwareComposer(Region& dirtyInOut);
+            void        setupHardwareComposer();
             void        composeSurfaces(const Region& dirty);