drm/display: bridge_connector: handle ycbcr_420_allowed

Follow the interlace_allowed example and calculate drm_connector's
ycbcr_420_allowed flag as AND of all drm_bridge's ycbcr_420_allowed
flags in a chain. This is one of the gaps between several
bridge-specific connector implementations and drm_bridge_connector.

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-v1-1-d74efac9e4e6@linaro.org


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