Add const to rate in cost_coeffs_b
Add const to rate in cost_coeffs_b

Change-Id: I9780e4e4f736b92b676f8a03a445a88749d4022e

GrokImageCompression/aom,smarter/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,smarter/aom,GrokImageCompression/aom,smarter/aom,smarter/aom,smarter/aom,GrokImageCompression/aom,smarter/aom