drm/amdgpu/powerplay: return success if set_mp1_state is not set

Some asics (APUs) don't have this callback so we want to return
success.  Avoids spurious error messages on APUs.

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