Fix part 1 of #37177 spikes in sculpting.

Was a typo from refactor to calculate sculpt plane from forward facing
vertices only. The branch of the code that did the calculation would end
up with a nice division by a wrong number.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@60900 954f8c5b-7b00-dc11-b283-0030488c597c