graphics/drm-fbsd12.0-kmod: Mark only for 12

Change the version check in graphics/drm-fbsd12.0-kmod to make it only
compile on FreeBSD 12, not 13 and later, since it fails to build on 12.0.
graphics/drm-current-kmod is the same driver versions, but with some
differences due to differences in 12 and current.

MFH: 2019Q3 (implicit, drm-drivers blanket)
Sponsored by: B3 Init