drm/tests: Add TDMS character rate connector state tests

drm/tests: Add TDMS character rate connector state tests

The previous patch stores in the connector state the expected TMDS
character rate matching the configuration of the HDMI connector. Let's
add a few 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-v15-12-c5af16c3aae2@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>