DoubleTap to Wake is a Function which is always Part of the Display-Firmware (when the Display support it).
Such Devices like, Nokia N9, some Limuas, Jolla 1, Jolla Tablet, Jolla C, Intex Aqua Fish, Aigo X86 and OnePlus X - has DoubleTap to Wake
Fairphone 2 unfortunately - not. So i think it will never find the way to the FP2