high bit depth (_hbd) and low bit depth (_lbd) versions
of the cfl functions: sum_above_row, sum_left_col,
cfl_build_prediction, cfl_luma_subsampling_420 (4:4:4 will
be added in subsequent commit) and cfl_alpha_dist. For
cfl_alpha_dist, special care is given to scale the SSE
according to the bit depth.

BUG=aomedia:835

Change-Id: I5b72845100d88fb8a438efe665bcae7fe1ba50b8