r600g/compute: Try to use a temporary resource when growing the pool

r600g/compute: Try to use a temporary resource when growing the pool

Now, before moving everything to host memory, we try to create a
new resource to use as a pool. I we succeed we just use this resource
and delete the previous one. If we fail we fallback to using the
shadow.

This should make growing the pool faster, and we can also save
64KB of memory that were allocated for the 'shadow', even if they
weren't used.

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