vp9/decode_tiles_mt: remove unnecessary local
vp9/decode_tiles_mt: remove unnecessary local

reuse the common loop index

Change-Id: I9db45a93c219c2123917514cb8e9d4ea86454711

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