Fix scores not using correct filename/display strings

I've updated all cases where we should have been using
`GetDisplayString()` anyway, but left the `ToString()` implementations
in place for safety. They should probably be removed in the future.

osu.Game/Scoring/ScoreInfo.cs

@@ -274,5 +274,7 @@ namespace osu.Game.Scoring
         }
 
         #endregion
+
+        public override string ToString() => this.GetDisplayTitle();
     }
 }