XGMI v4.8.0 is not used in any SOCs. Remove the associated functions.
Also, ensure get_xgmi_info callback pointer is not NULL before calling
the function.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>