anv: skip clip distance slots if were not requested
When computing the VUE map, in SSO mode (which is the one used in Vulkan), the slot for gl_ClipDistance is always reserved.
But later in the pipeline when emitting 3DSTATE_SBE we need to take this in account, because if the clip distance was not requested we need to skip it.
This fixes dEQP-VK.pipeline.max_varyings.*