By default, the DC_PRED is not cached (this includes
decoding). During cfl_rd_pick_alpha(), DC_PRED caching
is enabled, the DC_PRED is cached after the first time it
is computed (for each plane) and then it is reused when
testing all the other scaling parameters.

Change-Id: Ie8ba0bb0427c4d9be8de5b44e6330e8a78b9c7d9