r600g/compute: Defrag the pool if it's necesary

r600g/compute: Defrag the pool if it's necesary

This patch adds a new member to the pool to track its status.
For now it is used only for the 'fragmented' status, but if
needed it could be used for more statuses.

The pool will be considered fragmented if: An item that isn't
the last is freed or demoted.

This 'strategy' has a problem, although it shouldn't cause any bug.
If for example we have two items, A and B. We choose to free A first,
now the pool will have the 'fragmented' status. If we now free B,
the pool will retain its 'fragmented' status even if it isn't
fragmented.

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