drm/amd/display: Assume connectors are on single slot

[Why]
v1_5 display object table has no way for connectors to
indicate which slot they are a part of, resulting in additional
empty slots to appear in EDID management UI.

[How]
Assume that all connectors belong to the same slot.

Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Jaehyun Chung <jaehyun.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>