multi-res: disable intra on forced ref frames

If a reference frame is forced because of low dissimilarity, then
shut off the search of intra modes. This change has mixed results. On
one clip (QVGA), it hurt quality by ~1.5% with negligible speed impact.
On another (VGA) it had negligible affect on quality, but a ~0.2% speed
impact.

Change-Id: Ic8b07648979d732f489de5f094957e140f84d2eb