QTiffHandler: make two of the Private methods static

These two don't access *this, so save the this pointer when calling
them. Since their callers are (again) on the same class as the
methods, no name qualification is needed.

Pick-to: 6.12 6.11 6.8
Change-Id: Ic6a52a6ea988febdc7b49900ad3b83fa7b02f172
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>