The cost of signaling the alpha symbol and the signs are added to the
DC_PRED rate in RDO.

Results on Subset1(compared to f9e04152b with CfL enabled)

   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-0.3699 |  1.5330 |  0.8664 |  -0.2881 | -0.3107 | -0.2587 |     0.1954

Change-Id: Icd9827d11ee4ef29dfb527e636f0f380bcafa062