I believe you're right - you'll either want to turn it on from the ISR or you can set a flag in the ISR and, to keep it as short as possible, check for the flag in any polling function. In that function, you can do everything you need to do, including turning on the backlight. You might realise there's more to do than just turn on the backlight - you probably don't want the touch to be used to navigate somewhere since the user is touching a dark screen so you'll likely want to handle whether "touch just wakes up screen" vs "send touch coordinates to TouchGFX" logic there too.
