drm/mode_object: Export drm_mode_obj_find_prop_id for tests

We'll need to use drm_mode_obj_find_prop_id() for kunit tests to make
sure a given property has been properly created. Let's export it for
tests only.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240527-kms-hdmi-connector-state-v15-5-c5af16c3aae2@kernel.org


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