[CFL] separate uv_mode CDFs, split on is_cfl_allowed

This is to exclude coding CFL for block sizes where it is normatively
not allowed.

Reuse the alternate CDFs from the --disable-cfl path.

This yields some improvement on subset1:
   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-0.0113 | -0.2630 | -0.2138 |   0.0021 | -0.0110 | -0.0157 |    -0.0511

BUG=aomedia:1182

Change-Id: I1e5f23ef7f2debcf8edce788bfa63ec95db4a679