drm/bridge: lvds-codec: Constify the drm_bridge_funcs structure

The drm_bridge_funcs structure is never modified, make it const. Making
it read-only can improve security as the structure contains function
pointers.

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/20200224230056.2157-1-laurent.pinchart+renesas@ideasonboard.com