anv/allocator: Add support for a list of BOs in block pool.

anv/allocator: Add support for a list of BOs in block pool.

So far we use only one BO (the last one created) in the block pool. When
we switch to not use the userptr API, we will need multiple BOs. So add
code now to store multiple BOs in the block pool.

This has several implications, the main one being that we can't use
pool->map as before. For that reason we update the getter to find which
BO a given offset is part of, and return the respective map.

v3:
 - Simplify anv_block_pool_map (Jason).
 - Use fixed size array for anv_bo's (Jason)
v4:
 - Respect the order (item, container) in anv_block_pool_foreach_bo
 (Jason).

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>