D24829: Fix non-atomic output init

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REVISION SUMMARY
  The initial blank needs to happen after the output device has been
  initialized to get the current mode size.
  
  Also we can't assert in legacy mode on the page flip pending since
  a page flip can also be the result of a CRTC blank using a different.

TEST PLAN
  Full Plasma session
  
  BUG: 412684

REPOSITORY
  R108 KWin

BRANCH
  fixLegacyOutput

REVISION DETAIL
  https://phabricator.kde.org/D24829

AFFECTED FILES
  plugins/platforms/drm/drm_output.cpp

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