libdrm: drmNodeIsDRM: Add FreeBSD variant

FreeBSD devfs have on the gly generated major minor so we cannot use them
to test if the device is a drm node.
Instead get the devfs node name and test if it is in a subdirectory "drm/"
or "dri/".
Historycally DRM device on FreeBSD are created in /dev/drm/ and link are
present in /dev/dri/ for compatibility reason.

Signed-off-by: Emmanuel Vadot <manu@FreeBSD.org>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>