drm/amdgpu/pm: look up current_level for asics without pm callback

For asics without a callback, use the current level rather than 0xff.
This can avoid an unnecesary forced level set on older asics when
set by the user.

Reviewed-by: Kevin Wang <kevinyang.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>