Move variance functions to vpx_dsp

subpel functions will be moved in another patch.

Change-Id: Idb2e049bad0b9b32ac42cc7731cd6903de2826ce

vpx_dsp/sad.c

@@ -33,6 +33,7 @@ static INLINE unsigned int sad(const uint8_t *a, int a_stride,
   return sad;
 }
 
+// TODO(johannkoenig): this moved to vpx_dsp, should be able to clean this up.
 /* Remove dependency on vp9 variance function by duplicating vp9_comp_avg_pred.
  * The function averages every corresponding element of the buffers and stores
  * the value in a third buffer, comp_pred.