vp8: [loongson] optimize vp8_short_fdct4x4_mmi v2.
vp8: [loongson] optimize vp8_short_fdct4x4_mmi v2.

Optimize the calculate process of a,b,c,d.

Change-Id: I81717e47bc988ace1412d478513e7dd3cb6b0cc9

ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx