coef_probs: remove duplicate read/update code

Refactor per-transform copy & paste into a common function
update_coef_probs_common() and read_coef_probs_common(). The dry-run and
bit-writing loops in the encoder are still obvious candidates to be made
common, but they start to diverge a bit in the next commit, so are left
as-is for now.

Change-Id: I896bd3f4a073a6296eab7e92463fef79d8c6c08c