Fixed idct16x16_10 highbitdepth transform

In the case when there are only non-zero coefficients
in the first 4x4 block a special routine is called.
The highbitdepth optimized version of this routine
examined the wrong positions when deciding whether
to call an assembler or C inverse transform.

Change-Id: I62da663ca11775dadb66e402e42f4a1cb1927893