Chameleon: Increase the margin of pixel-diff for digital interfaces to 2

We saw some devices get a small amount of pixels with pixel-diff=2. As it
is a minor issue that an user does not notice, increasing the margin to
2 reduces some random failures.

BUG=chromium:438007
TEST=None, trivial change.

Change-Id: I6d20bba46f3b64e7d0812cda56bd3afd359762ab
Reviewed-on: https://chromium-review.googlesource.com/233346
Tested-by: Wai-Hong Tam <waihong@chromium.org>
Reviewed-by: Kalin Stoyanov <kalin@chromium.org>
Commit-Queue: Wai-Hong Tam <waihong@chromium.org>
1 file changed