D23602: [platforms/drm] Early platform and outputs teardown

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REVISION SUMMARY
  Since we now use in the backends the OutputDeviceInterface for output data
  all access must be complete before the Wayland server goes down. For that
  introduce a new function to prepare shutdown in the backends.
  
  While at it also remove the output deletion, since they get deleted through
  Qt's object system leading to crashes on double free.

TEST PLAN
  Shutdown works without seg faults in the Drm backend.

REPOSITORY
  R108 KWin

BRANCH
  fixDrmShutdown

REVISION DETAIL
  https://phabricator.kde.org/D23602

AFFECTED FILES
  main_wayland.cpp
  platform.cpp
  platform.h
  plugins/platforms/drm/drm_backend.cpp
  plugins/platforms/drm/drm_backend.h
  plugins/platforms/drm/drm_output.cpp

To: romangg, #kwin
Cc: kwin, LeGast00n, The-Feren-OS-Dev, sbergeron, jraleigh, fbampaloukas, GB_2, mkulinski, ragreen, jackyalcine, Pitel, iodelay, crozbo, bwowk, ZrenBot, ngraham, alexeymin, himcesjf, lesliezhai, ali-mohamed, hardening, romangg, jensreuterberg, abetts, sebas, apol, mart