gfio: fix problem with graph finding data range.

In finding minimum value, it used zero as an initializer
but if the graph had no data below say, 1000, it would still
find zero as the minimum, which was incorrect.

Signed-off-by: Stephen M. Cameron <scameron@beardog.cce.hp.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>