So far we only stopped compositing if all outputs were in dpms off
state. But it seems also to be possible that we don't have any outputs.
In that case our compositor would still try to render and at some point
this has to result in a crash.
To prevent such conditions this change set outputs as disabled also if
there are no outputs.
This is an attempt in fixing bug #402827. Unfortunately I cannot test as
I don't get into the condition with my notebook and also don't have an
AMD gpu whose driver seems to go into the no output conditions.