avcodec/mips: [loongson] optimize vp8 decoding in vp8dsp.

Optimize vp8 loop filter with mmi, four functions optimized:
1. ff_vp8_h_loop_filter8uv_mmi.
2. ff_vp8_v_loop_filter8uv_mmi.
3. ff_vp8_h_loop_filter16_mmi.
4. ff_vp8_v_loop_filter16_mmi.

Vp8 decoding speed improved about 50%(from 73fps to 110fps, Tested on loongson 3A3000).

Signed-off-by: Shiyou Yin <yinshiyou-hf@loongson.cn>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>