drm/bridge: fix kernel-doc warning in panel.c

drm/bridge: fix kernel-doc warning in panel.c

Add missing documentation to fix following warning:
panel.c:303: warning: Function parameter or member 'bridge' not described in 'drm_panel_bridge_connector'

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Boris Brezillon <boris.brezillon@collabora.com>
Cc: Mihail Atanassov <Mihail.Atanassov@arm.com>
Cc: Andrzej Hajda <a.hajda@samsung.com>
Cc: Neil Armstrong <narmstrong@baylibre.com>
Cc: Jonas Karlman <jonas@kwiboo.se>
Cc: Jernej Skrabec <jernej.skrabec@siol.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200328132025.19910-7-sam@ravnborg.org