drm/bridge: anx7625: Switch to devm MIPI-DSI helpers

Let's switch to the new devm MIPI-DSI function to register and attach
our secondary device.

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20211025151536.1048186-4-maxime@cerno.tech