drm/amd/display: Add MST link detection tests for connector

drm/amd/display: Add MST link detection tests for connector

Add KUnit coverage for amdgpu_dm_detect_mst_link_for_all_connectors()
to the amdgpu_dm_connector tests:

- No connectors registered: iteration body never runs and the call
  succeeds.
- Writeback connector: hit by the early continue
- Non-MST link
- MST branch without aux: NULL mst_mgr.aux

Assisted-by: Copilot:Claude-Opus-4.8
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Bhawanpreet Lakha <bhawanpreet.lakha@amd.com>
Signed-off-by: George Zhang <george.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>