xf86drm: use the correct device minor names on OpenBSD

Add defines for the device minor names and make use of them
in drmGetMinorName() so the correct paths will be used on OpenBSD.

v2: don't add new defines to xf86drm.h to keep them out of the API
    as requested by Emil.

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