drm_mm: extract check_free_mm_node

There are already two copies of this logic. And the new scanning
stuff will add some more. So extract it into a small helper
function.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Thomas Hellstrom <thellstrom@vmwgfx.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>