avcodec/mips: [loongson] reoptimize h264_chroma_mc8_mmi v2.

Reoptimize function ff_put_h264_chroma_mc8_mmi and ff_avg_h264_chroma_mc8_mmi.
Performance of h264 decoding improved about 5%(from 69fps to 73fps, tested on loongson 3A3000).
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>