Refactor the SSIM based rdmult update function
Refactor the SSIM based rdmult update function

Change-Id: I335103689659d9a2b291c4da54f07cdd9c2b1a6d

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