vk: Fix vkCreate/DestroyRenderPass

vk: Fix vkCreate/DestroyRenderPass

While updating vkDestroyObject, I discovered that vkDestroyPass reliably
crashes. That hasn't been an issue yet, though, because it is never
called.

In vkCreateRenderPass:
    - Don't allocate empty attachment arrays.
    - Ensure that pointers to empty attachment arrays are NULL.
    - Store VkRenderPassCreateInfo::subpassCount as
      anv_render_pass::subpass_count.

In vkDestroyRenderPass:
    - Fix loop bounds: s/attachment_count/subpass_count/
    - Don't call anv_device_free on null pointers.