zink/nir_to_spirv: emit ViewportIndex cap also for inputs
This fixes validation error VUID-VkShaderModuleCreateInfo-pCode-08737 for piglits:
spec@oes_viewport_array@viewport-gs-write-simple
spec@oes_viewport_array@viewport-gs-writes-in-range
spec@arb_fragment_layer_viewport@viewport-gs-no-write
spec@arb_fragment_layer_viewport@viewport-gs-write-simple
spec@arb_fragment_layer_viewport@viewport-gs-writes-in-range
spec@arb_fragment_layer_viewport@viewport-no-gs
spec@arb_fragment_layer_viewport@viewport-vs-write-gs-no-write-simple
spec@arb_fragment_layer_viewport@viewport-vs-write-simple