Remove hard-coded number for EXT_REFS
Remove hard-coded number for EXT_REFS

In the experiment of EXT_REFS, MAX_REF_FRAMES has increased from 4
to 7, as 3 more reference frames have been added.

Change-Id: I3fec2c2078f925afe518b454e3f71afbd7379d06

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