libdrm: Default to PCI for FreeBSD

FreeBSD have some support for DRM on !PCI device but no code is currently
upstream. Default to PCI for now.

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