drm/amdgpu: Add documentation for unique_id

Add the amdgpu.rst tie-ins for the unique_id documentation
Signed-off-by: Kent Russell <kent.russell@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

Documentation/gpu/amdgpu.rst

@@ -227,6 +227,12 @@ serial_number
 .. kernel-doc:: drivers/gpu/drm/amd/amdgpu/amdgpu_device.c
    :doc: serial_number
 
+unique_id
+---------
+
+.. kernel-doc:: drivers/gpu/drm/amd/amdgpu/amdgpu_pm.c
+   :doc: unique_id
+
 GPU Memory Usage Information
 ============================