vp9_cx_iface,encode: remove redundant cpi check
vp9_cx_iface,encode: remove redundant cpi check

this is allocated in init(), a NULL pointer on encode() should be
treated as a failure.

Change-Id: I3f55ea4a98be616bb5ab74d95ce4a3d6262e734a

mbebenita/aom,luctrudeau/aom,mbebenita/aom,smarter/aom,smarter/aom,Topopiccione/libvpx,mbebenita/aom,smarter/aom,Topopiccione/libvpx,ittiamvpx/libvpx-1,ittiamvpx/libvpx-1,mbebenita/aom,GrokImageCompression/aom,ShiftMediaProject/libvpx,smarter/aom,mwgoldsmith/libvpx,mwgoldsmith/libvpx,mbebenita/aom,mwgoldsmith/libvpx,ShiftMediaProject/libvpx,Topopiccione/libvpx,luctrudeau/aom,GrokImageCompression/aom,webmproject/libvpx,mwgoldsmith/vpx,smarter/aom,GrokImageCompression/aom,ShiftMediaProject/libvpx,Topopiccione/libvpx,luctrudeau/aom,ShiftMediaProject/libvpx,webmproject/libvpx,mbebenita/aom,webmproject/libvpx,GrokImageCompression/aom,mbebenita/aom,ittiamvpx/libvpx-1,mbebenita/aom,mwgoldsmith/vpx,smarter/aom,GrokImageCompression/aom,ittiamvpx/libvpx-1,mwgoldsmith/vpx,luctrudeau/aom,mwgoldsmith/vpx,webmproject/libvpx,mwgoldsmith/vpx,Topopiccione/libvpx,ittiamvpx/libvpx-1,ittiamvpx/libvpx-1,Topopiccione/libvpx,mwgoldsmith/libvpx,mwgoldsmith/libvpx,mwgoldsmith/vpx,luctrudeau/aom,mwgoldsmith/libvpx,webmproject/libvpx,webmproject/libvpx,GrokImageCompression/aom,luctrudeau/aom,mbebenita/aom,ShiftMediaProject/libvpx