drm/amd/powerplay: Add the interface for geting dpm current power state

implement the sysfs power_dpm_state

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