Extract the compution of the luma reconstructed average out of cfl_load
and into cfl_compute_average. The reconstructed luma average is stored
in the CFL_CONTEXT to avoid computing it for each transform block and
for each plane.

Results on subset1 (compared to 803bea26 with CfL)
   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-0.0474 | -0.1486 | -0.2931 |  -0.0358 | -0.0397 | -0.0127 |    -0.1162

Change-Id: I9e34af0fe5961ce8dbe70cb80aea2a16221d0d92