[CFL] Fix always use partition size to compute DC_PRED

plane_bsize is now computed properly. This also includes support for the
special case of blocks < 4X4

Results on subset1 (compared to 8e689e4b with CfL)
   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-0.0218 | -0.2328 | -0.2555 |  -0.0230 | -0.0379 | -0.0723 |    -0.1205

Change-Id: I6ec87d818d8df6a40ecf3bb1b86954e59c952930