refactor DM::SetupBitmap

Seemed sort of repetitive.

BUG=skia:
R=reed@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/286993005

git-svn-id: http://skia.googlecode.com/svn/trunk@14752 2bbb7eff-a529-9590-31e7-b0007b416f81
10 files changed