hwc: Skip dropped layers in video only composition

No need to draw dropped layers while doing video only composition
since they are lying outside updating ROI.

Change-Id: I7bef731538e39a5b16f17314ad31f0c4e7f2b618
diff --git a/libhwcomposer/hwc_mdpcomp.cpp b/libhwcomposer/hwc_mdpcomp.cpp
index d704749..774af7c 100644
--- a/libhwcomposer/hwc_mdpcomp.cpp
+++ b/libhwcomposer/hwc_mdpcomp.cpp
@@ -846,6 +846,7 @@
     int numAppLayers = ctx->listStats[mDpy].numAppLayers;
 
     mCurrentFrame.reset(numAppLayers);
+    mCurrentFrame.fbCount -= mCurrentFrame.dropCount;
     updateYUV(ctx, list, secureOnly);
     int mdpCount = mCurrentFrame.mdpCount;