Você já tentou editar o arquivo /etc/lightdm/lightdm.conf?
Como forçar múltiplos monitores de resoluções corretas para LightDM?
Por exemplo, adicionando
display-setup-script=/usr/share/lightdmxrandr.sh
executa o script acima na tela de login.
E adicionando
session-setup-script=/usr/share/lightdmxrandr.sh
executa o script acima durante a sessão.
No meu caso, esse script diz o seguinte:
#!/bin/bash
# connected external output
XCOM0='xrandr -q | egrep '(HDMI1|VGA1|DP1) connected' | egrep -o '(HDMI1|VGA1|DP1)''
# command to set external desktop
XCOM1='xrandr --output LVDS1 --primary --auto --output $XCOM0 --auto --right-of LVDS1'
# command to set normal desktop
XCOM2='xrandr --output LVDS1 --primary --auto'
# if the external monitor is connected, then we tell XRANDR to set up an extended desktop
if [ -n "$XCOM0" ] || [ ! "$XCOM0" = "" ]; then echo $XCOM1
# if the external monitor is disconnected, then we tell XRANDR to output only to the laptop screen
else echo $XCOM2
fi
exit 0;
# ERASE THESE COMMENTS:
# Forces multiple monitors correct resolutions for LightDM
# Tested with Ubuntu 12.04 LTS.
# Save this file as lightdmxrand.sh
# Set as an executable file like this:
# $ sudo chmod +x lightdmxrand.sh
# Move it to /usr/share/ directory like this:
# $ sudo mv lightdmxrand.sh /usr/share/lightdmxrandr.sh
# Edit /etc/lightdm/lightdm.conf adding the given line to it: display-setup-script=/usr/share/lightdmxrandr.sh
# If you want this script to also run after the login, add: session-setup-script=/usr/share/lightdmxrandr.sh
Espero que funcione!