Restore previous motion search bit-error scale.

The bit to error transformation got doubled as a result of going from
8-bit to 9-bit costs (change d13385ce).

Use defines to derive the scale numbers and comment some of the fields.

derf: -0.023 BDRATE
hevcmr: +0.067 BDRATE
stdhd: +0.098 BDRATE
(These are substantially smaller than than the original gains from 8 to
9 bit costing.)

Change-Id: I6a2b3b029b2f1415e4f90a05709b2333ec0eea9b