power: Fix display not blanking after gnome-shell wake up

- Let the machine's screen blank, or Win+L to lock it
- Screen turns off (mode "blank")
- Time passes, the sleep timeout kicks off
- We do nothing, as the action is "nothing" for sleep by default
- Notification comes in
- Screen wakes up (mode "normal") with a 15 seconds timeout
- 15 seconds timeout kicks off, we go back to the previous mode (mode "sleep")
- But mode "sleep" doesn't blank the screen, and does nothing, screen stays on

We'll fix this by never triggering the "sleep" idle mode when there's
nothing to do.