xf86drm: implement an OpenBSD specific drmGetDevice2

DRI devices on OpenBSD are not in their own directory.  They reside in
/dev with a large number of statically generated /dev nodes.

Avoid stat'ing all of /dev on OpenBSD by implementing this custom path.

v2:
   - use drmGetMinorType to get node type
   - adapt to drmProcessPciDevice changes
   - verify drmParseSubsystemType type is PCI
   - add a comment describing why this was added

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