drm/amd/display: Add subconnector property tests for connector

drm/amd/display: Add subconnector property tests for connector

Add KUnit coverage for update_subconnector_property() on the
amdgpu_dm_connector suite:

- DP connector with a sink: subconnector property is resolved from
  the dongle type (VGA converter -> VGA).
- DP connector without a sink: dongle type is ignored and the
  property stays Unknown.
- Non-DP connector: function early-returns and leaves a pre-seeded
  property value untouched.

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>