drm/amd/powerplay: optimize amdgpu_dpm_set_clockgating_by_smu() implementation

Cover the implementation details from outside(of power part).

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