drm/tests: hdmi: Fix WW_MUTEX_SLOWPATH failures

The light_up_connector helper function in the HDMI infrastructure unit
tests uses drm_atomic_set_crtc_for_connector(), but fails when it
returns an error.

This function can return EDEADLK though if the sequence needs to be
restarted, and WW_MUTEX_SLOWPATH is meant to test that we handle it
properly.

Let's handle EDEADLK and restart the sequence in our tests as well.

Fixes: eb66d34 ("drm/tests: Add output bpc tests")
Reported-by: Dave Airlie <airlied@gmail.com>
Closes: https://lore.kernel.org/r/CAPM=9tzJ4-ERDxvuwrCyUPY0=+P44orhp1kLWVGL7MCfpQjMEQ@mail.gmail.com/
Link: https://lore.kernel.org/r/20241031091558.2435850-1-mripard@kernel.org
Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20250129-test-kunit-v2-1-fe59c43805d5@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>

drivers/gpu/drm/tests/drm_hdmi_state_helper_test.c

@@ -70,10 +70,17 @@ static int light_up_connector(struct kunit *test,
 	state = drm_kunit_helper_atomic_state_alloc(test, drm, ctx);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, state);
 
+retry:
 	conn_state = drm_atomic_get_connector_state(state, connector);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, conn_state);
 
 	ret = drm_atomic_set_crtc_for_connector(conn_state, crtc);
+	if (ret == -EDEADLK) {
+		drm_atomic_state_clear(state);
+		ret = drm_modeset_backoff(ctx);
+		if (!ret)
+			goto retry;
+	}
 	KUNIT_EXPECT_EQ(test, ret, 0);
 
 	crtc_state = drm_atomic_get_crtc_state(state, crtc);