drm/amdgpu: initialize IP offset for navy_flounder

since navy_flounder has the same ip offset with sienna_cichlid,
follow sienna_cichlid setting for the moment.

Signed-off-by: Jiansong Chen <Jiansong.Chen@amd.com>
Reviewed-by: Tao Zhou <Tao.Zhou1@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>