drm/tests: Add HDMI TDMS character rate tests

The previous patch added an helper to compute the TMDS character rate on
an 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-v15-10-c5af16c3aae2@kernel.orgSigned-off-by: Maxime Ripard <mripard@kernel.org>