MFH: r562318 r562380 r562485

graphics/drm-fbsd13-kmod: Add new port

This is the drm port for FreeBSD 13.
Now that stable/13 is branched we can create it.

main (14-CURRENT) users still need to use either drm-current-kmod or
drm-devel-kmod

Fix PKGBASE collision

graphics/drm-fbsd13-kmod: Update comment

This is only supported on FreeBSD 13.

Reported by: rene