drm/amdgpu: Add xgmi API to set max speed/width

Add an API to set the max possible xgmi speed/width.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Asad Kamal <asad.kamal@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>