Further clean up of estimate_max_q.

Clean up and simplification of both estimate_max_q
variants and only call once per clip/section.

This leads to a more constrained range of Q values
across a clip / section.

Average gains across all 4 test sets:-

PSNR ~0.5%  SSIM ~0.3%

Change-Id: If77d5f7bb50939a464e117724f4da5b001c62d70