drm/bridge: Fix the bridge kernel doc

Commit 75146591 ("drm/bridge: Add a drm_bridge_state object")
introduced new helpers and hooks but the kernel was slightly broken.
Fix that now.

v2:
* Fix the drm_atomic_add_encoder_bridges() doc

Fixes: 75146591 ("drm/bridge: Add a drm_bridge_state object")
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200218151503.595825-1-boris.brezillon@collabora.com