libdrm: Add get_sysctl_pci_bus_info for FreeBSD

The FreeBSD drm driver expose a sysctl hw.dri.%d.busid which contain
the busid.
Use this sysctl to parse the busid information based on the major/minor
that allow us to implement FreeBSD support for drmParsePciBusInfo.

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