drm/bridge: display-connector: implement bus fmts callbacks

Since this bridge is tied to the connector, it acts like a passthrough,
so concerning the output & input bus formats, either pass the bus formats from the
previous bridge or return fallback data like done in the bridge function:
drm_atomic_bridge_chain_select_bus_fmts() & select_bus_fmt_recursive.

This permits avoiding skipping the negociation if the remaining bridge chain has
all the bits in place.

Without this bus fmt negociation breaks on drm/meson HDMI pipeline when attaching
dw-hdmi with DRM_BRIDGE_ATTACH_NO_CONNECTOR, because the last bridge of the
display-connector doesn't implement buf fmt callbacks and MEDIA_BUS_FMT_FIXED
is used leading to select an unsupported default bus format from dw-hdmi.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211020123947.2585572-2-narmstrong@baylibre.com

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