drm/amdgpu: ensure no PCIe peer access for CPU XGMI iolinks

[Why] Devices with CPU XGMI iolink do not support PCIe peer access.

Signed-off-by: Alex Sierra <alex.sierra@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>