[Gimli[2BV!2B]]

Hai provato a specificarla in xorg.conf?

Section "Device"
    Identifier "Configured Video Device"
    BusID      "PCI:0:12:0"
EndSection

Section "Monitor"
    Identifier "Configured Monitor"
EndSection

Section "Screen"
    Identifier "Default Screen"
    Monitor "Configured Monitor"
    Device "Configured Video Device"
EndSection