A separate prediction mode struct is added to allow
for uv-only modes (like CfL). Note: CfL will be
added as a separate mode in an upcoming commit.

Results on Subset1 (Compared to 4266a7ed 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

Change-Id: Ie80711c641c97f745daac899eadce6201ed97fcc