anv/meta: Allocate descriptor pools on-the-fly

We can't use a global descriptor pool like we were because it's not
thread-safe.  For now, we'll allocate them on-the-fly and that should work
fine.  At some point in the future, we could do something where we
stack-allocate them or allocate them out of one of the state streams.

src/intel/vulkan/anv_private.h

@@ -571,8 +571,6 @@ void anv_finish_wsi(struct anv_instance *instance);
 struct anv_meta_state {
    VkAllocationCallbacks alloc;
 
-   VkDescriptorPool desc_pool;
-
    /**
     * Use array element `i` for images with `2^i` samples.
     */