drm/amdgpu: PCI I/O bar can be disabled

PCI I/O bar can be disabled in VBIOS to save the resource. It is often
disabled in large aperture VBIOS. Don't call it an error.

Signed-off-by: Amber Lin <Amber.Lin@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>