Comment 13 for bug 924278

I got around this dpms-is-always-disabled nuisance
in Xubuntu 16.04 by calling a bash script
in ~/.config/autostart/<something>.desktop. The script
sleeps for 10 seconds and *then* calls xset in such a way
as to enable dpms. The 10-second delay
is evidently long enough for the startup of the X session to be
completed, along with the disablement of dpms that appears to happen during
session startup but, alas, after the contents of the autostart have been
processed.

I had earlier tried uninstalling light-locker, BTW, but it didn't help.