xf86drm: implement drmParseSubsystemType for OpenBSD

Implement drmParseSubsystemType for OpenBSD by always returning
DRM_BUS_PCI.  No non-pci drm drivers are in the kernel and this is
unlikely to change anytime soon as the existing ones aren't permissively
licensed.

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>