How to enable "double tap to wake up" in any android device?
It can be enabled only if your ROM/ kernel support that feature. To check if it is enabled download 3C Toolbox from Play Store , go to System Manager &rarr; Miscellaneous  and you will have an option that shows up to enable double tap feature, IF it is supported. Figure shows it is not supported on my device

I am not aware if CM supports enabling this feature using the inbuilt settings