drm/tests: Add test for drm_atomic_helper_check_modeset()

Add a test for drm_atomic_check_modeset() specifically to validate
drm_atomic_check_valid_clones() helper

Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20241216-concurrent-wb-v4-4-fe220297a7f0@quicinc.com


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