drm: remove #define's for non-linux systems

There is no point in considering FreeBSD et al.  in the linux kernel
source code.

Signed-off-by: Carlos R. Mafra <crmafra@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>