avcodec/mips: [loongson] simplify the usage of intermediate variable addr.

Simplify the usage of intermediate variable addr in following functions:
1. ff_put_pixels4_8_mmi
2. ff_put_pixels8_8_mmi
3. ff_put_pixels16_8_mmi
4. ff_avg_pixels16_8_mmi.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>