loginDialog: reset auth prompt on vt switch before fade in

At the moment, if a user switches to the login screen vt,
the login screen fades in whatever was on screen prior, and
then does a reset.

It makes more sense to reset first, so we fade in what the
user is going to interact with instead of what they interacted
with before.

js/gdm/loginDialog.js

@@ -917,6 +917,7 @@ var LoginDialog = GObject.registerClass({
     }
 
     _loginScreenSessionActivated() {
+        this._authPrompt.reset();
         Tweener.addTween(this,
                          { opacity: 255,
                            time: _FADE_ANIMATION_TIME,
@@ -929,11 +930,7 @@ var LoginDialog = GObject.registerClass({
                                        children[i].opacity = this.opacity;
                                }
                            },
-                           onUpdateScope: this,
-                           onComplete() {
-                               this._authPrompt.reset();
-                           },
-                           onCompleteScope: this });
+                           onUpdateScope: this });
     }
 
     _loginScreenSessionDeactivated() {