Either by chance, or by misread, the current evaluation interval may be
zero. If that is the case, don't divide by it!
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170218112708.24504-1-chris@chris-wilson.co.ukReviewed-by: NRadoslaw Szwichtenberg <radoslaw.szwichtenberg@intel.com>