backends/drm: disable triple buffering on NVidia by default
There's some unresolved glitches on multi gpu systems. Until that's taken care of, triple buffering is disabled by default on NVidia unless the environment variable KWIN_DRM_DISABLE_TRIPLE_BUFFERING=0 is set
(cherry picked from commit 0133431c)
Co-authored-by: Xaver Hugl xaver.hugl@gmail.com