iPhone screen pin layout
The screen would have a backlight.  You need to find the pins which power this part - no need to power the LCD in front of the backlight.  

Here's an example of actually controlling the LCD, so it may also provide info on the backlight controls: https://www.youtube.com/watch?v=vCWGMM00ZEk

Do a quick search on "iphone screen pinout" and you will get lots of info.  Also you may have to input a PWM or other signal to adjust the backlight brightness.