power: Correctly put the screen and computer to sleep when idle

The idle timer was only being set up when dimming was enabled. By default on AC
this is disabled, and this meant the x_idle boolean flag never got set.
This meant the display or computer sleep timeouts were then never set.

By using the dim timeout and just ignoring the state change if no display dim
is enabled we make sure that the computer and display timeouts are set correctly.

Additionally, make sure that we check for the suspend inhibit when we want to
suspend, rather than when we setup the timers, as new inhibits may be present.

Resolves https://bugzilla.gnome.org/show_bug.cgi?id=659066