drm_hwcomposer: Use proper commit for author/committer check
The script uses the author/committer of HEAD instead of the commit it is inspecting. This fails when a patch set has different authors/committers (such as !46 (merged))
Change-Id: I0fcd724cf372fad435c7614777f13e015c204c3d Signed-off-by: Sean Paul seanpaul@chromium.org