drm/amdgpu: initialize IP offset for sienna_cichlid (v2)

Add IP offset headers and state.

V2: squash in updates (Alex)

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>