libdrm: Implement drmParsePciDeviceInfo for FreeBSD

The FreeBSD kernel expose a pseudo-device /dev/pci to obtain information
about present PCI device.
Uee the PCIOCGETCONF ioctl on this device to look up the desired device
information.

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