logind: allow suspending if there are no displays

logind: allow suspending if there are no displays

With proprietary graphics drivers, there won't be any 'drm' devices in
sysfs, so logind will never suspend the system upon closing the lid,
even if only one (internal) display is connected. This has been reported
by multiple users so far.

IMHO, it's better to suspend the system in this case for safety reasons,
to avoid having nvidia blob users' laptops overheat, for the same reason
that sleep inhibitors are overridden (LidSwitchIgnoreInhibited=yes).