Increase YV12BufferPow2 test threshold

The NPOT version already has 3 as the threshold and at least one
platform seems to have diff of 3 in one of the internal pixels for POW2
variant.

Bug: 21306103
Bug: 30920650
Change-Id: I7882a6ff43ffc862d95fea32c8ee8e7f19fb759d
Cherry-pick from master (e3747fd2)