drm/mediatek: mtk_dsi: Rename bridge to next_bridge

This is really a cosmetic change just to make a bit more readable the
code after convert the driver to drm_bridge. The bridge variable name
will be used by the encoder drm_bridge, and the chained bridge will be
named next_bridge.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>