From ArchWiki

if nvidia, don't we need

Option "DPMS" "true"

so something like this:

Section "Device"
   Identifier  "nvidia"
   Driver      "nvidia"
   Option	"NoLogo" "true"
   Option	"DPMS" "true"
   #VideoRam    131072
   # Insert Clocks lines here if appropriate
EndSection

??

also, I think we need to disable xscreensaver and gnome screensaver or other screensaver