Ensure minimum exec width of the PME OpenCL kernels

Ensure minimum exec width of the PME OpenCL kernels

This change adds checks to make sure that we don't execute incorrect
kernels in the case of the rare event if the Intel OpenCL compiler
decides to generate spread or gather kernels for 8-wide execution.

Refs #2516 #2520

Change-Id: I7ab33accebe908a56eb194e8245dfcfa6f817324