So drivers don't need dummy functions just returning NULL.

Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Reviewed-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: NOleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20180529135918.19729-1-kraxel@redhat.com