drm/amd/powerplay: tag swSMU code layers

Per designs, the swSMU code is separated into four layers. And the typical
calling flow should be like: amdgpu_smu.c -> ${asic}_ppt.c -> smu_v11/12_0.c
-> smu_cmn.c. Compile errors will come out for any violations. This can
help to prevent cross callings(e.g. amdgpu_smu.c -> ${asic}_ppt.c ->
amdgpu_smu.c -> ${asic}_ppt.c) which were common in our code.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>