[0.85] clean displaypref + factorization

git-svn-id: https://forge.indepnet.net/svn/glpi/branches/0.85-work@21261 35612d4c-9d10-0410-837c-bf9dd1be60d7
[0.85] forget in previous commit

git-svn-id: https://forge.indepnet.net/svn/glpi/branches/0.85-work@21262 35612d4c-9d10-0410-837c-bf9dd1be60d7