drm/bridge: dw-hdmi: set mtmdsclock for deep color

Configure the correct mtmdsclock for deep colors to prepare support
for 10, 12 & 16bit output.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Jernej Škrabec <jernej.skrabec@siol.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200304104052.17196-2-narmstrong@baylibre.com