anv/pipeline: Fix requirement for depthstencil state

anv/pipeline: Fix requirement for depthstencil state

The Vulkan spec allows VkGraphicsPipelineCreateInfo::pDepthStencilState
to be NULL when the pipeline's subpass contains no depthstencil
attachment (see spec quote below). anv_pipeline_init_dynamic_state()
required it unconditionally.

This path fixes anv_pipeline_init_dynamic_state() to access
pDepthStencilState only when there is a depthstencil attachment.

From the Vulkan spec (20 Oct 2015, git-aa308cb)

   pDepthStencilState [...] may only be NULL if renderPass and subpass
   specify a subpass that has no depth/stencil attachment.