anv: Lower ViewIndex to zero when multiview is disabled
Vulkan spec says
If multiview is enabled in the render pass, this value will be one
of the bits set in the view mask of the subpass the pipeline is
compiled against. If multiview is not enabled in the render pass,
this value will be zero.
Closes: #4446 (closed) Fixes: 0db70703 ("anv/pipeline: Add shader lowering for multiview")