D17985: [platforms/drm] Mark outputs as disabled if we don't have any outputs

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REVISION SUMMARY
  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.
  
  CCBUG: 402827

TEST PLAN
  Completely untested

REPOSITORY
  R108 KWin

BRANCH
  drm-no-outputs-disable

REVISION DETAIL
  https://phabricator.kde.org/D17985

AFFECTED FILES
  plugins/platforms/drm/drm_backend.cpp
  plugins/platforms/drm/drm_backend.h

To: graesslin, #kwin
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