drm/bridge: dw-hdmi: truly enforce 420-only formats when drm mode demands it

The current output bus format selection logic is enforcing YUV420 even
when the drm mode allows for other bus formats as well.
Fix it by adding check for 420-only drm modes.

Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Acked-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/6e6a217c180584a67ed7992c785764ba54af9151.1687702042.git.adrian.larumbe@collabora.com