how many hours has my laptop display been used?
If you're turning the brightness all the way down, the display is still on, but the backlight for the display is off. I do not believe a backlight tracker exists.

I do not believe a display tracker exists either.

However, assuming you rarely or never use your MBP with the screen closed (needs external keyboard/mouse/screen), you can calculate a rough estimate by multiplying the battery's cycle count by the average lifespan of the battery (100%-0%). I'm not sure why this information would be relevant, but for example if I have a cycle count of 460 and my battery generally lasts for about 7 hours, I'd have a display count of (very roughly) about 3200 hours.