drm: mark dma setup/teardown as legacy systems

And hide the checks a bit better. This was already disallowed for
modesetting drivers, so no functinal change here.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>