drm/amd/pp: Implement get_performance_level for legacy dgpu

display can get clock info through this function.
implement this function for vega10 and old asics.
from vega12, there is no power state management,
so need to add new interface to notify display
the clock info

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>