drm/amdgpu: add psp command to get num xgmi links between direct peers

The TA can now be invoked to provide the number of xgmi links connecting
a direct source and destination peer.
Non-direct peers will report zero links.

Signed-off-by: Jonathan Kim <jonathan.kim@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>