backends/drm: block the colorspace capability by default on NVidia
There have been many reports that enabling HDR on NVidia can lead to KWin's atomic commits being rejected on login, which means KWin can't display anything. To prevent users from falling into that trap, this commit disables the colorspace capability unless the KWIN_DRM_ALLOW_NVIDIA_COLORSPACE=1 environment variable is set. This commit should be reverted once the driver problem causing the issue has been fixed.