drm: replace DRM_COPY macro w/ a function

Don't inline it; the compiler can figure it out.  Comments added that are
based upon my interpretation of the code.  Hopefully they're correct. :)

Signed-off-by: Andres Salomon <dilinger@collabora.co.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>