drm/bridge: lvds-codec: Add to_lvds_codec() function

Factor out the manual container_of() uses to a common to_lvds_codec()
macro to shorten lines (and provide better type safety, although that
won't matter much in this case).

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200224225645.28060-1-laurent.pinchart+renesas@ideasonboard.com