drm/mm: Remove MM_UNUSED_TARGET

This was missed in

commit c700c67b
Author: David Herrmann <dh.herrmann@gmail.com>
Date:   Sat Jul 27 13:39:28 2013 +0200

    drm/mm: remove unused API

Cc: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>