drm/amd/display: Create amdgpu_dm_wb_connector

[WHY]
We need to track the dc_link and it would get confusing if
re-using the amdgpu_dm_connector.

[HOW]
Creating new amdgpu_dm_wb_connector.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>