Comment 30 for bug 874241

Workaround i found in -> https://bugs.launchpad.net/ubuntu/+source/unity-greeter/+bug/1082952/comments/6

1) log in
2) use xrandr or the Displays control utility to configure your monitors how you'd like them to be configured in the login screen
3) copy ~/.config/monitors.xml to /var/lib/lightdm/.config

If resolution alway resets after reboot try to do in Gnome 3
 Settings -> Displays -> Arrange Combined Displays ( the same as Step 2)