drm/bridge: dw-hdmi: set bridge's ycbcr_420_allowed flag

Set the drm_bridge's ycbcr_420_allowed flag if the YCbCr 420 output is
supported by the hardware.

Cc: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20241019-bridge-yuv420-v1-6-d74efac9e4e6@linaro.org


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>