d3d12: Support PIPE_CAP_VS_LAYER_VIEWPORT

While helping debug !23114 (merged) I noticed we're not setting this cap, even though we could be. D3D12 always supports these bits being exported from the VS, even if it means that the underlying driver might need to insert an implicit GS. Whether they're natively supported is determined by everyone's favorite cap: VPAndRTArrayIndexFromAnyShaderFeedingRasterizerSupportedWithoutGSEmulation.