drm/tests: connector: Add ycbcr_420_allowed tests

drm/tests: connector: Add ycbcr_420_allowed tests

Extend HDMI connector output format tests to verify its registration
succeeds only when the presence of YUV420 in the supported formats
matches the state of ycbcr_420_allowed flag.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20241224-bridge-conn-fmt-prio-v4-4-a9ceb5671379@collabora.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>