Benchmarks/QGraphicsItem: use testdata instead hard-coded values

tst_QGraphicsItem::setPos() was provided with test data
but it was not used.

Change-Id: I8ed2a1ef9940024e4a0e666276f0953706869ef7
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

tests/benchmarks/widgets/graphicsview/qgraphicsitem/tst_qgraphicsitem.cpp

@@ -154,7 +154,7 @@ void tst_QGraphicsItem::setPos()
     processEvents();
 
     QBENCHMARK {
-        rect->setPos(10, 10);
+        rect->setPos(pos);
     }
 }