drm/tests: Add test for drm_atomic_helper_commit_modeset_disables()

Add a subtest to check that modeset is called when the connector is
changed

Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20241211-abhinavk-modeset-fix-v3-2-0de4bf3e7c32@quicinc.com


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>