graphics/drm-515-kmod: re-add check for __FreeBSD_version < 1400081

Build breakages on FreeBSD 13 started to show up without any
other changes to the sources. Turns out when the port was disabled
for FreeBSD 16 and newer, the check for FreeBSD pre-1400081 was removed.

Re-add that check to avoid constant build failure reports.

Fixes: eef56dc01bd7
Reviewed by: manu, jrm
Differential Revision: https://reviews.freebsd.org/D54813