First pass clean up.
One of a series of changes to clean up two pass allocation as precursor to support for multiple arf or boosted frames per GF/ARF group. This change pulls out the calculation of the total bits allocated to a GF/ARF group into a function, to aid readability and reduce the line count for define_gf_group(). This change should have no material impact on output. Change-Id: I716fba08e26f9ddde3257e7d9b188453791883a3