Fix missing update of QTreeView on setTreePosition()

There was missing update in setTreePosition() or better to say update
was but in not that part of QTreeView that needed. Now QTreeView
correctly updates and paints tree decoration in a new place on
changing tree position.

[ChangeLog][QtWidgets][QTreeView] Fixed missing update of QTreeView
on changing tree position.

Task-number: QTBUG-65980
Change-Id: Id79ab8fcb39d511245a551068640684dd2a10cb9
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>

src/widgets/itemviews/qtreeview.cpp

@@ -987,7 +987,7 @@ void QTreeView::setTreePosition(int index)
 {
     Q_D(QTreeView);
     d->treePosition = index;
-    update();
+    d->viewport->update();
 }
 
 /*!