r600g/compute: Implement compute_memory_demote_item

r600g/compute: Implement compute_memory_demote_item

This function will be used when we want to map an item
that it's already in the pool.

v2: Use temporary variables to avoid so many castings in functions,
    as suggested by Tom Stellard

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