The code introduced abs_diff() as migrated from drm to standard
linux headers. The LINUXKPI_VERSION check did not allow this to
be visible by default in-tree. Make sure the v6.6 check is optional.
drm-kmod always defines LINUXKPI_VERSION so this should be fine.
Sponsored by: The FreeBSD Foundation
Fixes: afc450fac9f04
MFC after: 3 days