Changed segmentation check order

In SPLITMV, the 8x8 segment will be checked first.  If the 8x8 rd
is better than the best, we check the other segments.  Otherwise
bail.  Adjustments to the thresh_mult were necessary to make
up for the initial quality loss.
The performance improved by 20% (average) for good quality,
speed 0 and speed 1, while the overall quality remained the same.

Change-Id: I717aef401323c8a254fba3e9777d2a316c774cc3