libgui: Fix CPU rendering on Surface

When the surface damage code went in, it incorrectly assumed that if
an application was doing CPU rendering, it would be using lock and
unlockAndPost instead of dequeue and queue, so it repurposed the dirty
region too aggressively. This change keeps it from clobbering the
dirty region if a CPU producer is attached.

Bug: 20431815
Change-Id: Id4dfd71378311ea822f0289f6de2d20a7bd84014
diff --git a/include/gui/Surface.h b/include/gui/Surface.h
index 8217652..a9f78cf 100644
--- a/include/gui/Surface.h
+++ b/include/gui/Surface.h
@@ -298,12 +298,12 @@
     sp<GraphicBuffer>           mPostedBuffer;
     bool                        mConnectedToCpu;
 
-    // In the lock/unlock context, this reflects the region that the producer
-    // wished to update and whether the Surface was able to copy the previous
-    // buffer back to allow a partial update.
+    // When a CPU producer is attached, this reflects the region that the
+    // producer wished to update as well as whether the Surface was able to copy
+    // the previous buffer back to allow a partial update.
     //
-    // In the dequeue/queue context, this reflects the surface damage (the
-    // damage since the last frame) passed in by the producer.
+    // When a non-CPU producer is attached, this reflects the surface damage
+    // (the change since the previous frame) passed in by the producer.
     Region mDirtyRegion;
 };