drm/meson: meson_dw_hdmi: add bridge and switch to drm_bridge_funcs

Switch the dw-hdmi driver to drm_bridge_funcs by implementing a new local
bridge, connecting it to the dw-hdmi bridge, then implement the
atomic_get_input_bus_fmts/atomic_get_output_bus_fmts.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Jernej Škrabec <jernej.skrabec@siol.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200304104052.17196-8-narmstrong@baylibre.com