performance improvement

Improved hashrate on both mtp.cl and mtp_vega.cl

mtp.cl for RX5xx and RX4xx

1.69MH @1250MH/z on RX580
do not use this kernel for vega

mtp_vega
1.685MH @1250MHz on RX580
3.1MH/s @1350MHz on Vega 56