Look in 

/etc/X11/xdm/Xsetup
/etc/X11/xdm/Xsetup_0
/etc/X11/xdm/Xreset
/etc/X11/xdm/Xreset_0

Put something like 

xset -dpms
xset s off

in one or some or all of these files. 

-chris



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