drm/bridge: ps8640: Switch to devm MIPI-DSI helpers

Let's switch to the new devm MIPI-DSI function to register and attach
our secondary device. This also avoids leaking the device on removal.

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20211025151536.1048186-12-maxime@cerno.tech