drm/bridge: lt8912b: Switch to devm MIPI-DSI helpers

[ Upstream commit 1fdbf66e

 ]

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/20211025151536.1048186-6-maxime@cerno.tech
Stable-dep-of: 941882a0

 ("drm/bridge: lt8912b: Fix bridge_detach")
Signed-off-by: Sasha Levin <sashal@kernel.org>