How do I prevent the proprietary Linux ATI driver from detecting my screen's dpi?

How do I prevent the proprietary Linux ATI driver from detecting my screen's dpi?

The proprietary ATI driver for Linux tends to misdetect dpi when I plug in different monitors (plugging into a TV over HDMI after using with a normal DVI monitor). Either it correctly detects it has a low dpi (and fonts are unreadably small) or it thinks it has a very high dpi (and fonts are absurdly large). How do I tell it to always use a specific dpi?