avcodec/mips: [loongson] fix bug of svq3-watermark failed in fate test.

Failed case: svq3-watermark
When minimum loop count of following functions are greater than parameter h passed to them, svq3-watermark failed.
1. ff_put_pixels4_8_mmi
2. ff_avg_pixels4_8_mmi
3. ff_put_pixels4_l2_8_mmi
4. ff_avg_pixels4_l2_8_mmi
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>