BQ: Prevent operations on disconnected BQs

- Update unit tests to match

Bug 23763412

Change-Id: I77e59bf6b57b328433c3835450455f80a8fa454b
diff --git a/services/surfaceflinger/DisplayHardware/VirtualDisplaySurface.cpp b/services/surfaceflinger/DisplayHardware/VirtualDisplaySurface.cpp
index 94f30d5..0880a44 100644
--- a/services/surfaceflinger/DisplayHardware/VirtualDisplaySurface.cpp
+++ b/services/surfaceflinger/DisplayHardware/VirtualDisplaySurface.cpp
@@ -476,7 +476,8 @@
     return NO_ERROR;
 }
 
-void VirtualDisplaySurface::cancelBuffer(int pslot, const sp<Fence>& fence) {
+status_t VirtualDisplaySurface::cancelBuffer(int pslot,
+        const sp<Fence>& fence) {
     if (mDisplayId < 0)
         return mSource[SOURCE_SINK]->cancelBuffer(mapProducer2SourceSlot(SOURCE_SINK, pslot), fence);