avcodec/mips: loongson optimize mmi load and store operators

avcodec/mips: loongson optimize mmi load and store operators

1.MMI_ load/store macros are defined in libavutil/mips/mmiutils.h
2.Replace some unnecessary unaligned access with aligned operator
3.The MMI_ load/store is compatible with cpu loongson2e/2f which not support instructions start with gs

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