drm/encoder: remove obsolete documentation of bridge

In commit 05193dc3 ("drm/bridge: Make the bridge chain a
double-linked list") the bridge has been removed and replaced by a
private field. Remove the leftover documentation of the removed field.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200911135413.3654800-2-m.tretter@pengutronix.de