drm/atomic-helper: fix kerneldoc

Just a tiny copypasta mistake.

Fixes: 75146591 ("drm/bridge: Add a drm_bridge_state object")
Cc: Boris Brezillon <boris.brezillon@collabora.com>
Acked-by: Boris Brezillon <boris.brezillon@collabora.com>
Cc: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200214081340.2772853-1-daniel.vetter@ffwll.ch