Hid nbnxn kernel names behind a lookup function

Keeps clang syntax checking in master branch quiet - otherwise
it complains about unused variables. See discussion in
Change-Id: I753f701d74b8d4533e74cede19712b030bf97ca6

Change-Id: Id73ef041d7d3c9443be4992dc18f28ac09f499e9
Merge "Hid nbnxn kernel names behind a lookup function" into release-4-6