Move update of ref frame probabilities in encode loop.

The existing code updated the reference frame probabilities before
the test to evaluate the impact of using updated probabilities
in vp8_estimate_entropy_savings().

The estimate of cost and savings is still basic and does not reflect
the new prediction code but this would require per MB costings
and the benefit is probably marginal, as this is really just used for
rate estimation in the loop.

Change-Id: Id6ba88ae6e11c273b3159deff70980363ccd8ea1