drm/tests: Add tests for Broadcast RGB property

This had a bunch of kunit tests to make sure our code to handle the
Broadcast RGB property behaves properly.

This requires bringing a bit of infrastructure to create mock HDMI
connectors, with custom EDIDs.

Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240527-kms-hdmi-connector-state-v15-19-c5af16c3aae2@kernel.org


Signed-off-by: Maxime Ripard <mripard@kernel.org>