Don't warn or fail if it's missing.

v2: handle xgmi case more gracefully.
v3: handle older kernels properly
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Tested-by: James Zhu <James.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>