LCL: fixed TCustomForm.MoveToDefaultPosition finding the monitor from form pos

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@39871 4005530d-fff6-0310-9dd1-cebe43e6787f
LCL: simplified TCustomForm.MoveToDefaultPosition

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@39872 4005530d-fff6-0310-9dd1-cebe43e6787f