drm/bridge: synopsys: dw-hdmi: add bus format negociation

Add the atomic_get_output_bus_fmts, atomic_get_input_bus_fmts to negociate
the possible output and input formats for the current mode and monitor,
and use the negotiated formats in a basic atomic_check callback.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Jernej Škrabec <jernej.skrabec@siol.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200304104052.17196-5-narmstrong@baylibre.com