anv/allocator: Add a start_offset to anv_state_pool

anv/allocator: Add a start_offset to anv_state_pool

This allows a pool's allocations to start somewhere other than the base
address.  Our first real use of this will be to use a negative offset
for the binding table pool to make it so that the offset is baked into
the pool and the code in anv_batch_chain.c doesn't have to understand
pool offsetting.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4897>