libvpx

Corrected optimization of 8x8 DCT code

The 8x8 DCT uses a fast version whenever possible.
There was a mistake in the checking code which
meant sometimes the fast version was used when it
was not safe to do so.

Change-Id: I154c84c9e2d836764768a11082947ca30f4b5ab7
(cherry picked from commit fd05fb0c21e253b4d6f92d7e0b752850ff8ab188)