drm/amdgpu: Deprecate xgmi_link_speed enum

xgmi doesn't have discrete max speeds defined. Speed numbers can be
arbitrary based on SOC. Deprecate the enum.

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>