drm/amdgpu: use VCN firmware offset for cache window

Since we are using the signed FW now, and also using PSP firmware loading,
but it's still potential to break driver when loading FW directly
instead of PSP, so we should add offset.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>