drm/bridge: dw-hdmi: add support for YUV420 output

In order to support the HDMI2.0 YUV420 display modes, this patch
adds support for the YUV420 TMDS Clock divided by 2 and the controller
passthrough mode.

YUV420 Synopsys PHY support will need some specific configuration table
to support theses modes.

This patch is based on work from Zheng Yang <zhengyang@rock-chips.com> in
the Rockchip Linux 4.4 BSP at [1]

[1] https://github.com/rockchip-linux/kernel/tree/release-4.4



Cc: Zheng Yang <zhengyang@rock-chips.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1549022873-40549-5-git-send-email-narmstrong@baylibre.com