anv/allocator: Tweak the block pool growing algorithm

anv/allocator: Tweak the block pool growing algorithm

The old algorithm worked fine assuming a constant block size.  We're
about to break that assumption so we need an algorithm that's a bit more
robust against suddenly growing by a huge amount compared to the
currently allocated quantity of memory.

Reviewed-by: Juan A. Suarez Romero <jasuarez@igalia.com>