drm/amdgpu/vcn:Correct VCN cache window definition

Correct VCN cache window definition. The old one
is reused from UVD, and it is not fully correct.

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