Also, move body of update_cfl_costs() to av1_fill_mode_rates().

Results on Subset1 (Compared to 1cfe474b with CFL enabled)

  PSNR | PSNR Cb | PSNR Cr | PSNR HVS |   SSIM | MS SSIM | CIEDE 2000
0.0000 |  0.0000 |  0.0000 |   0.0000 | 0.0000 |  0.0000 |     0.0000

No change in bitstream, for an average encode speed-up of 2.3%.

Change-Id: I3948abcd70cfecad8086edfe4c45552b576ae06f