drm/tests: hdmi: handle empty modes in find_preferred_mode()

If the connector->modes list is empty, then list_first_entry() returns a
bogus entry. Change that to use list_first_entry_or_null().

Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20241130-hdmi-mode-valid-v5-1-742644ec3b1f@linaro.org


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