Make ref_mv_idx syntax context dependent on block distance only
Make ref_mv_idx syntax context dependent on block distance only

This allows the hardware decoder to start decoding ref_mv_idx
syntax prior to the sorting stage and hide the latency of entropy
decoding. The compression performance change is about 0.01% level.

Change-Id: I48d9878df337d053d597d96c153ef444399a5115

luctrudeau/aom,mbebenita/aom,luctrudeau/aom,mbebenita/aom,mbebenita/aom,smarter/aom,luctrudeau/aom,smarter/aom,GrokImageCompression/aom,smarter/aom,smarter/aom,GrokImageCompression/aom,mbebenita/aom,luctrudeau/aom,GrokImageCompression/aom,GrokImageCompression/aom,mbebenita/aom,mbebenita/aom,mbebenita/aom,smarter/aom,GrokImageCompression/aom,mbebenita/aom,luctrudeau/aom,luctrudeau/aom,GrokImageCompression/aom,smarter/aom,mbebenita/aom