drm/amd/pp: Fix bug that dpm level was not really locked

Lock the dpm levels when we use SW method to modify
the dpm tables directly to avoid a possible race
with the smu.

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>