graphics/drm-current-kmod: Adjust FreeBSD compat

Update the FreeBSD OSVERSION compat of drm-current-kmod after the latest
update in r502043.

Suggested by: jbeich
Discussed with: johalun
Aproved by: jmd (maintainer, implicit)
MFH: 2019Q2 (implicit, drm-drivers blanket)
Sponsored by: B3 Init