Improving model rd with variance and quant step

Improves the rd modeling function and implements them using interpolation
from a table which is a little faster. Also uses sse as input to the
modeling function rather than var - since there is no dc prediction
used and as a result the sse works a little better.

derfraw300: +0.05%
Speedup: ~1%

Change-Id: I151353c6451e0e8fe3ae18ab9842f8f67e5151ff