drm/bridge: Drop conditionals around of_node pointers

This patch is based on commit c9e358df ("driver-core: remove
conditionals around devicetree pointers").

Having conditional around the of_node pointer of the drm_bridge
structure turns out to make driver code use ugly #ifdef blocks. Drop the
conditionals to simplify drivers. While this slightly increases the size
of struct drm_bridge on non-OF system, the number of bridges used today
and foreseen tomorrow on those systems is very low, so this shouldn't be
an issue.

So drop #if conditionals by adding struct device_node forward declaration.

Suggested-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230831080938.47454-3-biju.das.jz@bp.renesas.com