r600g/compute: Add statuses to the compute_memory_items

r600g/compute: Add statuses to the compute_memory_items

These statuses will help track whether the items are mapped
or if they should be promoted to or demoted from the pool

v2: Use the new is_item_in_pool util function,
    as suggested by Tom Stellard

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