drm/bridge: dw-hdmi: add max bpc connector property

Add the max_bpc property to the dw-hdmi connector to prepare support
for 10, 12 & 16bit output support.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jernej Škrabec <jernej.skrabec@siol.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200304104052.17196-3-narmstrong@baylibre.com