Added code to prevent I8X8_PRED mode for MBs using 8x8 transform

This fixed a conflict introduced by the change of adding 8x8 intra
prediction modes. The 8x8 intra prediction mode code assumed the
use of 4x4 transform, and causes encoder crashes when the codec is
configured with --enable-t8x8.

Change-Id: I00cc94df63e9725377ffba9eb51be6b77fe3fcf9