drm/tests: modes: Fix drm_display_mode memory leak

drm_analog_tv_mode() and its variants return a drm_display_mode that
needs to be destroyed later one. The
drm_test_modes_analog_tv_mono_576i() test never does however, which
leads to a memory leak.

Let's make sure it's freed.

Reported-by: Philipp Stanner <[e-mail]>
Closes: https://lore.kernel.org/dri-devel/[e-mail]/
Fixes: e31538489908 ("drm/tests: Add tests for the new Monochrome value of tv_mode")
Reviewed-by: Thomas Zimmermann <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Maxime Ripard <[e-mail]>

drivers/gpu/drm/tests/drm_modes_test.c

···
 {
 	struct drm_test_modes_priv *priv = test->priv;
 	struct drm_display_mode *mode;
+	int ret;
 
 	mode = drm_analog_tv_mode(priv->drm,
 				  DRM_MODE_TV_MODE_MONOCHROME,
 				  13500 * HZ_PER_KHZ, 720, 576,
 				  true);
 	KUNIT_ASSERT_NOT_NULL(test, mode);
+
+	ret = drm_kunit_add_mode_destroy_action(test, mode);
+	KUNIT_ASSERT_EQ(test, ret, 0);
 
 	KUNIT_EXPECT_EQ(test, drm_mode_vrefresh(mode), 50);
 	KUNIT_EXPECT_EQ(test, mode->hdisplay, 720);