PERF: qMRMLSceneDisplayableModel: Factor out constant display node lookup.

This whole section of code is confusing and seems redundant, but it has
been refactored many times and everyone left it there so leave alone.
For now we can at least avoid repeated lookups and save a few %.

From: Isaiah Norton <inorton@bwh.harvard.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25050 3bd1e089-480b-0410-8dfb-8563597acbee