drm/amdgpu/pm: document power_dpm_state

This is a legacy file and is only provided for
backwards compatibility.

Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>