LinuxKPI: correct the LINUXKPI_VERSION check for abs_diff()

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 (initially)
Fixes: afc450fac9f04
MFC after: 3 days
Reviewed by: dumbbell
Differential Revision: https://reviews.freebsd.org/D52071