hwc: Don't use yuv count for fb Z order.

In video-only cases, don't use yuv count for fb Z order, instead use
the mdp count which indicates how many video layers are actually
being sent to overlays.

Change-Id: I53ab4316fbb17f5d054066ef2dd85d89183d6a8c
diff --git a/libhwcomposer/hwc_mdpcomp.cpp b/libhwcomposer/hwc_mdpcomp.cpp
index 720b1c6..4396256 100644
--- a/libhwcomposer/hwc_mdpcomp.cpp
+++ b/libhwcomposer/hwc_mdpcomp.cpp
@@ -778,7 +778,7 @@
         //Destination over
         mCurrentFrame.fbZ = -1;
         if(mCurrentFrame.fbCount)
-            mCurrentFrame.fbZ = ctx->listStats[mDpy].yuvCount;
+            mCurrentFrame.fbZ = mCurrentFrame.mdpCount;
 
         mCurrentFrame.map();
         if(!programYUV(ctx, list)) {