Do in-place UV intra mode selection.

This means we only do UV intra mode selection if we find any intra
mode to actually be useful at all; in addition, we only do UV intra
mode selection for the transform sizes that were selected, rather
than all sizes available in this partition.

First 50 frames of bus @ 1500kbps (speed 0) gains about 5% with this
change.

Change-Id: I7b461eb8b803247f57896c5a9505f745b55502b3