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
BUG: 494547
BUG: 494407
Also see https://invent.kde.org/plasma/kwin/-/merge_requests/
Edited by Xaver Hugl