avcodec/mips: [loongson] reoptimize put and add pixels clamped functions.

Simplify the usage of intermediate variable addr and remove unused variable all64
in following functions:
1. ff_put_pixels_clamped_mmi
2. ff_put_signed_pixels_clamped_mmi
3. ff_add_pixels_clamped_mmi

This optimization speed up mpeg4 decode about 2% on loongson platform(tested with 3A3000).
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>