Fixing External Monitor Display Settings in Game Mode

I accidentally set my external HDMI display settings to 1440x900 @ 85 Hz using the Display Settings in game mode. Unlike desktop mode, there's no confirmation dialog when you mess up like this, and so now I can't use my external display anymore in game mode.

Has anybody else done this? Is there a way to force game mode to reset to default settings for an external display? Does anybody know where in the file system these settings are kept?