xf86drm: correct the OpenBSD DRM_MAJOR define

As far as I can tell no OpenBSD platform ever used 81
for a drm major.  While the value was added to libdrm in 2003
or earlier drm didn't appear in OpenBSD till 2007.

Of the OpenBSD platforms that support drm amd64/macppc/sparc64
use a major of 87, i386 uses 88.

v2: rearrange ifdefs as suggested by Emil.

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