drm/bridge: lt9611: Register and attach our DSI device at probe

In order to avoid any probe ordering issue, the best practice is to move
the secondary MIPI-DSI device registration and attachment to the
MIPI-DSI host at probe time. Let's do this.

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Tested-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20211025151536.1048186-9-maxime@cerno.tech