iris: Use thread safe slab allocators in transfer_map handling

pipe->transfer_map can be called from u_threaded_context's thread
rather than the driver thread.  We need to use two different slab
allocators, one for each thread.  transfer_unmap, on the other hand,
is only ever called from the driver thread.

Reviewed-by: Zoltán Böszörményi <zboszor@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8964>

src/gallium/drivers/iris/iris_context.h

@@ -576,6 +576,9 @@ struct iris_context {
    /** Slab allocator for iris_transfer_map objects. */
    struct slab_child_pool transfer_pool;
 
+   /** Slab allocator for threaded_context's iris_transfer_map objects */
+   struct slab_child_pool transfer_pool_unsync;
+
    struct blorp_context blorp;
 
    struct iris_batch batches[IRIS_BATCH_COUNT];