drm/tests: Add output bpc tests

Now that we're tracking the output bpc count in the connector state,
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>
Tested-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Link: https://patchwork.freedesktop.org/patch/msgid/20240527-kms-hdmi-connector-state-v15-6-c5af16c3aae2@kernel.org


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