avcodec/mips: [loongson] mmi optimizations for VP9 put and avg functions
VP9 decoding speed improved about 60.5%(from 38fps to 61fps, tested on loongson 3A3000).
Reviewed-by: Shiyou Yin <yinshiyou-hf@loongson.cn>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>