Fix TableView::positionViewAtRow()

Task-number: QTBUG-31631
Change-Id: Iaf5bd5c895f0fd94c7ffec19b571786ee9352782
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>