drm/amd/powerplay: implement sysfs of get num states function

add function smu_get_power_num_state function for sw-smu.
it's only for sysfs interface, the power state feature is not ready for sw-smu.

Signed-off-by: Kevin Wang <Kevin1.Wang@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>