Further rate control cleanups

Further rate control cleanups

Includes various cleanups.
Streamlines the interfaces so that all rate control state
updates happen in the vp9_rc_postencode_update() function.
This will hopefully make it easier to support multiple
rate control schemes.
Removes some unnecessary code, which in rare cases can casue
a difference in the constrained quality mode output, but
other than that there is no bitstream change yet.

Change-Id: I3198cc37249932feea1e3691c0b2650e7b0c22fc