drm/vc4: dpi: Switch to devm_drm_of_get_bridge

The new devm_drm_of_get_bridge removes most of the boilerplate we
have to deal with. Let's switch to it.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210910130941.1740182-3-maxime@cerno.tech