drm/amdgpu/nbio: update vcn doorbell range

VCN4.0.3 used up to 16 doorbells per partition.

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>