intel/vulkan: Fix clear_depth_stencil_attachment with unused attachment
When the depth_stencil_attachment is VK_ATTACHMENT_UNUSED, then subpass->depth_stencil_attachment, so check that for NULL before trying to dereference it.
When the depth_stencil_attachment is VK_ATTACHMENT_UNUSED, then subpass->depth_stencil_attachment, so check that for NULL before trying to dereference it.