Replace incorrectly removed background logic

I had earlier thought that removing this would solve the weirdness with
starting the beatmap on any but the first difficulty, and forgot to
replace it when I found the actual cause.

osu.Game/Screens/Play/Player.cs

@@ -251,6 +251,9 @@ namespace osu.Game.Screens.Play
         protected override void OnEntering(GameMode last)
         {
             base.OnEntering(last);
+            
+            (Background as BackgroundModeBeatmap)?.BlurTo(Vector2.Zero, 1000);
+            Background?.FadeTo((100f- dimLevel)/100, 1000);
 
             Content.Alpha = 0;
             dimLevel.ValueChanged += dimChanged;