drm/bridge: Fix atomic state ops documentation

The drm_bridge_funcs atomic_state_duplicate and atomic_state_destroy
operations are erroneously documented as having a default implementation
if not implemented in bridge drivers. This isn't correct, fix the
documentation.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200226112514.12455-6-laurent.pinchart@ideasonboard.com