Fix compiler warnings when simply including qgraphicsscene.h

Fix compiler warnings when simply including qgraphicsscene.h

Commit 02b80ebb52b957ae65cc23f5c9464633d9de868d marked itemAt(x, y) and
itemAt(point) as deprecated, causing a warning when code calls the function.
Unfortunately the itemAt(x, y) overload calls the itemAt(point) overload,
causing a compiler warning even when not using any functions from
qgraphicsscene.h at all.

This commit fixes itemAt(x, y) to not call itemAt(point) but use the
non-deprecated items() function directly.

Change-Id: I9c4d3a070d65f3406cdab00345c56d4fc62e76c1
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>