r600g/compute: Defrag the pool at the same time as we grow it

r600g/compute: Defrag the pool at the same time as we grow it

This allows us two things: we now need less item copies when we have
to defrag+grow the pool (to just one copy per item) and, even in the
case where we don't need to defrag the pool, we reduce the data copied
to just the useful data that the items use.

Note: The fallback path is a bit ugly now, but hopefully we won't need
it much.

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