avcodec/mips: [loongson] optimize memset in h264dsp.

Optimized memset with mmi in following functions:
1. ff_h264_add_pixels4_8_mmi.
2. ff_h264_idct_add_8_mmi.
3. ff_h264_idct8_add_8_mmi.

This optimization improved h264 decoding performance about 1.3%(tested on loongson 3A3000).

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>