hwc: set mModeOn flag for fbupdate properly

Set mModeOn flag for fbupdate to false if validateAndSet function
fails, so that queueBuffer can be avoided for prepare failure.

Change-Id: Ifcc7a16caf8779f13411ae2a9eeeec6e6c846f4c
diff --git a/libhwcomposer/hwc_fbupdate.cpp b/libhwcomposer/hwc_fbupdate.cpp
index bf2aee5..678e090 100644
--- a/libhwcomposer/hwc_fbupdate.cpp
+++ b/libhwcomposer/hwc_fbupdate.cpp
@@ -66,8 +66,9 @@
 bool IFBUpdate::prepareAndValidate(hwc_context_t *ctx,
             hwc_display_contents_1 *list, int fbZorder) {
     hwc_layer_1_t *layer = &list->hwLayers[list->numHwLayers - 1];
-    return prepare(ctx, list, layer->displayFrame, fbZorder) &&
+    mModeOn = prepare(ctx, list, layer->displayFrame, fbZorder) &&
             ctx->mOverlay->validateAndSet(mDpy, ctx->dpyAttr[mDpy].fd);
+    return mModeOn;
 }
 
 //================= Low res====================================