Revert "Only construct luma predictors once per block."
Revert "Only construct luma predictors once per block."

This reverts commit 1919c303d66611e8578b9c732cb33cca32377b11.

The predictor is constructed per plane, and we unfortunately don't have
access to the other planes in this function.

luctrudeau/daala,KyleSiefring/daala,HeadhunterXamd/daala,felipebetancur/daala,felipebetancur/daala,ycho/daala,xiphmont/daala,felipebetancur/daala,nvoron23/daala,ycho/daala,HeadhunterXamd/daala,jmvalin/daala,mbebenita/daala,ycho/daala,nvoron23/daala,kodabb/daala,kustom666/daala,KyleSiefring/daala,jmvalin/daala,nvoron23/daala,HeadhunterXamd/daala,kodabb/daala,tribouille/daala,KyleSiefring/daala,kodabb/daala,mbebenita/daala,kustom666/daala,KyleSiefring/daala,felipebetancur/daala,xiph/daala,mbebenita/daala,kodabb/daala,nvoron23/daala,xiph/daala,xiphmont/daala,felipebetancur/daala,kustom666/daala,luctrudeau/daala,HeadhunterXamd/daala,xiphmont/daala,mbebenita/daala,mbebenita/daala,luctrudeau/daala,kustom666/daala,luctrudeau/daala,jmvalin/daala,KyleSiefring/daala,kodabb/daala,ycho/daala,tribouille/daala,ycho/daala,tribouille/daala,luctrudeau/daala,tribouille/daala,jmvalin/daala,jmvalin/daala,HeadhunterXamd/daala,xiph/daala,xiphmont/daala,xiph/daala,xiphmont/daala,kustom666/daala,tribouille/daala,xiph/daala,nvoron23/daala