vp9: Rate control fix for CBR mode.
vp9: Rate control fix for CBR mode.

For CBR mode: modify the qp clamping to allow q to respond
faster to overshoot. Can reduce some suprious overshoot events
observed in screen content coding.

Change-Id: I0b3f54b0d1b4086182f834e557a4121950b176d4

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