avcodec: loongson optimize h264dsp idct and loop filter with mmi

Change-Id: Ic87fb8f5cd22a502ff9dbbc5a5a8ea97cfc8a1dd
Signed-off-by: ZhouXiaoyong <zhouxiaoyong@loongson.cn>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
