Fixed calling of highbd transform.

This patch does not change behaviour because
vp9_fwht4x4 is identical to vp9_highbd_fwht4x4,
but it becomes important when accelerations are made
to vp9_highbd_fwht4x4 in a later patch.

Change-Id: I2b790316cdd498727c4951a9e591edb291de3ac8
Merge "Fixed calling of highbd transform." into highbitdepth