QPainter: Fix (!a == b) bug

The ! binds to a only, which is most likely wrong here.

Change-Id: I4aa09407a8acc68f97bef474781193774e4a7597
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>
Reviewed-by: Kurt Pattyn <pattyn.kurt@gmail.com>