Xft antialias settings must not override requested NoAntialias

If Xft enabled font antialiasing, QFont::NoAntialias would have no
effect as it would be overridden.

Change-Id: I4dae264bc6674ae81f181cc9ce85851174d42544
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>