Rename the function rd_pick_best_sub8x8_mode()
Rename the function rd_pick_best_sub8x8_mode()

This large function is solely used for the RDO search for
inter prediction mode. It would be helpful for readers if its name
tells that whole function is used for inter mode decision only.

Change-Id: Ida366b142b7129bf89498227d186c54341c3af5e

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