Add enum pp_xgmi_plpd_mode to describe PLPD policies.

v2: move the enum from amdgpu_smu.h to kgd_pp_interface.h
Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Asad Kamal <asad.kamal@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>