drm/amdgpu: prepare the output buffer for GET_PEER_LINKS command

Per the xgmi ta implementation, KGD needs to fill in node_ids
in concern into the shared command output buffer rather than the
command input buffer.

Input buffer is not used for GET_PEER_LINKS command execution.

In this way, xgmi ta can reuse the node info in the output buffer
just filled in and populate the same buffer with link info directly.

Signed-off-by: Shiwu Zhang <shiwu.zhang@amd.com>
Reviewed-by: Le Ma <le.ma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>