The only difference between the two was that the vp9 function allowed
for every step in the bilinear filter (16 steps) while vp8 only allowed
for half of those. Since all the call sites in vp9 (<< 1) the input, it
only ever used the same steps as vp8.

This will allow moving the subpel variance to vpx_dsp with the rest of
the variance functions.

Change-Id: I6fa2509350a2dc610c46b3e15bde98a15a084b75