drm/tests: hdmi: Setup ycbcr_420_allowed before initializing connector

drm/tests: hdmi: Setup ycbcr_420_allowed before initializing connector

Initializing HDMI connector via drmm_connector_hdmi_init() requires its
->ycbcr_420_allowed flag to be adjusted according to the supported
formats passed as function argument, prior to the actual invocation.

In order to allow providing test coverage for YUV420 modes, ensure the
flag is properly setup.

Reviewed-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Link: https://lore.kernel.org/r/20250527-hdmi-conn-yuv-v5-12-74c9c4a8ac0c@collabora.com
Signed-off-by: Maxime Ripard <mripard@kernel.org>