anv/allocator: Rework state streams again

anv/allocator: Rework state streams again

If we're going to hav valgrind verify state streams then we need to ensure
that once we choose a pointer into a block we always use that pointer until
the block is freed.  I was trying to do this with the "current_map" thing.
However, that breaks down because you have to use the map from the block
pool to get to the stream_block to get at current_map.  Instead, this
commit changes things to track the stream_block by pointer instead of by
offset into the block pool.