xf86drm: open correct render node on non-linux

drm render nodes have the same major as drm primary devices but offset
the minor by a base of 128.

I expected the name of the device to have numbering starting at 0 when
these non-linux codepaths were added (before OpenBSD had render nodes).

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Acked-by: Eric Engestrom <eric.engestrom@intel.com>