drm/tests: Add RGB Quantization tests

The previous commit added the infrastructure to the connector state to
track what RGB Quantization should be used in a given state for an HDMI
connector.

Let's add some kunit tests to make sure it works as expected.

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-v15-21-c5af16c3aae2@kernel.org


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