[CFL] Avoid uselessly calling av1_predict_intra_block
[CFL] Avoid uselessly calling av1_predict_intra_block

For chroma DC_PRED, cfl_predict_block and av1_predict_intra_block are
redundant,

Results on Subset1 (compared to  e1c27392 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: Id50f3682eaccb5370e29d66af8a9967a30d135e6

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom