drm/amdgpu: correct ras enabled flag

XGMI RAS should be according to the gmc xgmi physical nodes number,
XGMI RAS should not be enabled if xgmi num_physical_nodes is zero.

Signed-off-by: Stanley.Yang <Stanley.Yang@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>