drm/amdgpu: make sure ddc_bus is valid in connector unregister

This should only happen on boards TV connectors which do not
have a ddc bus for those connectors.  None of the asics supported
by amdgpu support tv, so we shouldn't hit this, but check
to be on the safe side (e.g., bios bug for example).

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>