[WHY] Writeback mode tests call drm_mode_probed_add without holding mode_config.mutex, producing a warning for every generated mode. [HOW] Hold the mutex around writeback mode enumeration and release it before checking the returned count or inspecting the generated modes. Assisted-by: Copilot:GPT-6-Astra Reviewed-by: Bhawanpreet Lakha <bhawanpreet.lakha@amd.com> Signed-off-by: Alex Hung <alex.hung@amd.com> Signed-off-by: George Zhang <george.zhang@amd.com> Tested-by: Dan Wheeler <daniel.wheeler@amd.com> Signed-off-by: Alex Deucher <alexander.deucher@amd.com>