Always reset layer acquireFenceFd after commit

If SurfaceFlinger needs to refresh the screen but the dirty region is
empty, it won't set the layer acquire fences, and stale file
descriptors will be passed to HWC commit(). Now we make sure to clear
the stale file descriptors for each layer right after commit().

Bug: 7078301
Change-Id: I6953ff91fc5488f105b30b07306f9c45a4c3f780

services/surfaceflinger/DisplayHardware/HWComposer.cpp

@@ -598,6 +598,8 @@ public:
             visibleRegion.numRects = 0;
             visibleRegion.rects = NULL;
         }
+
+        getLayer()->acquireFenceFd = -1;
     }
 };