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