vp8: restrict 1st pass cpu_used range
vp8: restrict 1st pass cpu_used range

< 4 isn't meaningful in the first pass; additional analysis will be
done, but thrown out, unnecessarily increasing the runtime.

Change-Id: Ic3de77e3eaa7a8a3371f76f84693e9655c60fdba

webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx