anv/allocator: Rework chunk return to the state pool.

anv/allocator: Rework chunk return to the state pool.

This commit tries to rework the code that split and returns chunks back
to the state pool, while still keeping the same logic.

The original code would get a chunk larger than we need and split it
into pool->block_size. Then it would return all but the first one, and
would split that first one into alloc_size chunks. Then it would keep
the first one (for the allocation), and return the others back to the
pool.

The new anv_state_pool_return_chunk() function will take a chunk (with
the alloc_size part removed), and a small_size hint. It then splits that
chunk into pool->block_size'd chunks, and if there's some space still
left, split that into small_size chunks. small_size in this case is the
same size as alloc_size.

The idea is to keep the same logic, but make it in a way we can reuse it
to return other chunks to the pool when we are growing the buffer.

v2:
 - Include Jason's suggestions to the algorithm that returns chunks.
 - Update comments.

v3:
 - Disallow returning 0 blocks (Jason).
 - fix min_size in the loop (Jason).
 - remove temporary variables (Jason)
v4:
 - return_chunk() should never return blocks larger than
 pool->block_size.

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