drm: Document how to register devices without struct drm_bus

With the recent addition of the drm_set_unique() function, devices can
now be registered without requiring a drm_bus. Add a brief description
to the DRM docbook to show how that can be achieved.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>