drm: bridge: icn6211: Implement atomic_get_input_bus_fmts

Implement .atomic_get_input_bus_fmts callback, which sets up the
input (DSI-end) format, and that format can then be used in pipeline
format negotiation between the DSI-end of this bridge and the other
component closer to the scanout engine.

Acked-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Jagan Teki <jagan@amarulasolutions.com>
Cc: Maxime Ripard <maxime@cerno.tech>
Cc: Robert Foss <robert.foss@linaro.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
To: dri-devel@lists.freedesktop.org
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220331150509.9838-9-marex@denx.de