drm/meson: dw-hdmi: stop enforcing input_bus_format

To allow using formats from negotiation, stop enforcing input_bus_format
in the private dw-plat-data struct.

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/20200304104052.17196-9-narmstrong@baylibre.com