Optimize theora decoding with mmi in functions:
1. ff_vp3_idct_add_mmi
2. ff_vp3_idct_put_mmi
3. ff_vp3_idct_dc_add_mmi
4. ff_put_no_rnd_pixels_l2_mmi

Theora decoding speed improved about 32%(from 88fps to 116fps, Tested on loongson 3A3000).
Reviewed-by: Shiyou Yin <yinshiyou-hf@loongson.cn>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>