Another fix of daala-dist for cb4x4

Daala-dist replaces the luma distortion of sub8x8 partitions with
its own distortion thus requires to split the luma distortion only.
Doing so, there has been a bug that INT_MAX64 value comes
when the sub8x8 parition is skipped. This happened because the existing
code does not initialize the rd_stats_y or tmp_rd_stats_y, i.e. rd_stat struct
for luma only in several places.

Change-Id: If229b53bb7a6cff0b8751138a32b1dcf02665624