drm/bridge: ti-sn65dsi86: Implement bridge->mode_valid()

For the brave new world of bridges not creating their own connectors, we
need to implement the max clock limitation via bridge->mode_valid()
instead of connector->mode_valid().

v2: Drop unneeded connector->mode_valid()

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210920225801.227211-3-robdclark@gmail.com