r600g/compute: Remove unneeded code from compute_memory_promote_item

r600g/compute: Remove unneeded code from compute_memory_promote_item

Now that we know that the pool is defragmented, we positively know
that allocated + unallocated will be the total size of the
current pool plus all the items that will be promoted. So we only
need to grow the pool once.

This will allow us to just add the new items to the end of the
item_list without the need of looking for a place to the new item.

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>