If softpin is supported, create new BOs for the required size and add the
respective BO maps. The other main change of this commit is that
anv_block_pool_map() now returns the map for the BO that the given
offset is part of. So there's no block_pool->map access anymore (when
softpin is used.

v3:
 - set fd to -1 on softpin case (Jason)

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