drm/radeon/kms: free fence IB if it wasn't emited at IB free time

If at IB free time fence wasn't emited that means the IB wasn't
scheduled because an error occured somewhere, thus we can free
then fence and mark the IB as free.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>