drm/amdgpu: Add xgmi speed/width related info

Add APIs to initialize XGMI speed, width details and get to max
bandwidth supported. It is assumed that a device only supports same
generation of XGMI links with uniform width.

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>