D24051: [platforms/drm] introduce env var to select GPU

bshah created this revision.
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REVISION SUMMARY
  In some environemnts it is possible that udev detection of primary GPU
  fails and it picks the first available card (normally card0). But it is
  not desired, and it would be required to use different card (card1).
  
  Introduce environment variable, which when used, will make it possible
  to use different card node then one selected by udev.
  
  You can use it like KWIN_DRM_DEVICE=/dev/dri/card1 kwin_wayland

TEST PLAN
  Tested on the device where card0 was not usable for kms, but card1

REPOSITORY
  R108 KWin

BRANCH
  bshah/env-for-drmdev

REVISION DETAIL
  https://phabricator.kde.org/D24051

AFFECTED FILES
  plugins/platforms/drm/drm_backend.cpp

To: bshah, #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, mart