drm/radeon: fix the r100/r200 ums block 0 page fix

commit cf22f20a upstream.

airlied -> brown paper bag.

I blame Hi-5 or the Wiggles for lowering my IQ, move the fix inside some
brackets instead of breaking everything in site.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>