drm: bridge: Constify mode arguments to bridge .mode_set() operation

The mode and ajusted_mode passed to the bridge .mode_set() operation
should never be modified by the bridge (and are not in any of the
existing bridge drivers). Make them const to make this clear.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>