background: fix multimonitor background placement in overview

This is like the previous commit but for monitor index, instead
of effects.

https://bugzilla.gnome.org/show_bug.cgi?id=696712

js/ui/background.js

@@ -242,6 +242,7 @@ const BackgroundCache = new Lang.Class({
             this._loadImageContent({ filename: params.filename,
                                      style: params.style,
                                      effects: params.effects,
+                                     monitorIndex: params.monitorIndex,
                                      cancellable: params.cancellable,
                                      onFinished: params.onFinished });