perf.cfg: update indexes to compute averages

We just adjusted the title order in the result, so
the avg_update indexes also need to be updated.

tpkt_per_exit 13=6/11,rpkt_per_irq 12=5/10 according
to following columns.

2             3       4              5       6
throughput    CPU     thr_per_CPU    rx_pkts tx_pkts
7         8        9        10       11
rx_byts   tx_byts  re_pkts  irq_inj  io_exit
12           13
rpkt_per_irq tpkt_per_exit

Signed-off-by: Amos Kong <akong@redhat.com>