Refactor REFS_PER_FRAME definition
Refactor REFS_PER_FRAME definition

Define it as a function of reference frame types to provide
scalability for multiple reference frames.

Change-Id: I77b856c96916f352bc31004b9266b3f24e19bd0f

ittiamvpx/libvpx-1,mbebenita/aom,ShiftMediaProject/libvpx,running770/libvpx,luctrudeau/aom,mwgoldsmith/vpx,Topopiccione/libvpx,webmproject/libvpx,jmvalin/aom,ittiamvpx/libvpx-1,jmvalin/aom,webmproject/libvpx,zofuthan/libvpx,zofuthan/libvpx,mbebenita/aom,jmvalin/aom,shacklettbp/aom,jmvalin/aom,mwgoldsmith/libvpx,thdav/aom,openpeer/libvpx_new,felipebetancur/libvpx,zofuthan/libvpx,felipebetancur/libvpx,mbebenita/aom,mwgoldsmith/libvpx,shacklettbp/aom,liqianggao/libvpx,mbebenita/aom,smarter/aom,luctrudeau/aom,ittiamvpx/libvpx-1,liqianggao/libvpx,Topopiccione/libvpx,mbebenita/aom,VTCSecureLLC/libvpx,ShiftMediaProject/libvpx,felipebetancur/libvpx,shacklettbp/aom,zofuthan/libvpx,liqianggao/libvpx,GrokImageCompression/aom,VTCSecureLLC/libvpx,ittiamvpx/libvpx-1,smarter/aom,jmvalin/aom,luctrudeau/aom,GrokImageCompression/aom,ittiamvpx/libvpx-1,Topopiccione/libvpx,thdav/aom,running770/libvpx,luctrudeau/aom,mwgoldsmith/libvpx,VTCSecureLLC/libvpx,running770/libvpx,GrokImageCompression/aom,webmproject/libvpx,luctrudeau/aom,Topopiccione/libvpx,running770/libvpx,smarter/aom,thdav/aom,smarter/aom,felipebetancur/libvpx,VTCSecureLLC/libvpx,webmproject/libvpx,thdav/aom,running770/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/vpx,luctrudeau/aom,mwgoldsmith/vpx,mbebenita/aom,jmvalin/aom,mbebenita/aom,zofuthan/libvpx,mwgoldsmith/libvpx,openpeer/libvpx_new,mwgoldsmith/vpx,mbebenita/aom,shacklettbp/aom,liqianggao/libvpx,webmproject/libvpx,shacklettbp/aom,liqianggao/libvpx,ShiftMediaProject/libvpx,GrokImageCompression/aom,GrokImageCompression/aom,smarter/aom,mwgoldsmith/vpx,thdav/aom,ittiamvpx/libvpx-1,GrokImageCompression/aom,openpeer/libvpx_new,webmproject/libvpx,mbebenita/aom,ShiftMediaProject/libvpx,Topopiccione/libvpx,thdav/aom,openpeer/libvpx_new,mwgoldsmith/vpx,openpeer/libvpx_new,felipebetancur/libvpx,shacklettbp/aom,running770/libvpx,smarter/aom,zofuthan/libvpx,mwgoldsmith/libvpx,openpeer/libvpx_new,VTCSecureLLC/libvpx,mwgoldsmith/libvpx,VTCSecureLLC/libvpx,liqianggao/libvpx,Topopiccione/libvpx,felipebetancur/libvpx