Comment 26 for bug 160871

It's using DDC (Display Data Channel) to get the EDID (Extended Display Identification Data). DDC is the method by which an EDID is obtained.
The EDID is list of supported modes and features from the monitor. Generally LVDS panels only support one resolution: the native mode of the panel. Other modes are supported by scaling a non-native mode to the native mode of the panel using the scaling hardware. In the cases where a monitor provides an EDID, we use that data since it should contain the monitor's preferred modelines. In monitors without an EDID, we attempt to add the modes in the display subsection of screen section of the config.

You can add new modes at run time using xrandr:
xrandr --newmode 1024x768 63.50 1024 1072 1176 1328 768 771 775 798 -hsync +vsync
xrandr --addmode LVDS 1024x768