tests/checkasm/checkasm:

                          C          LSX       LASX
hevc_idct_32x32_8_c:      1243.0     211.7     101.7

Speedup of decoding H265 4K 30FPS 30Mbps on
3A6000 with 8 threads is 1fps(56fps-->57fps).

Reviewed-by: yinshiyou-hf@loongson.cn
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>