Ensure PME with OpenCL does not attempt to pin

Ensure PME with OpenCL does not attempt to pin

Host-only memory pinning was designed with CUDA in mind, while OpenCL
requires managing both host and device memory buffer for efficient
mapping, which is not yet implemented.

This change teaches the PME module to understand what pinning policy
is appropriate to the build configuration, so that the setup of data
structures in various parts of the code can use a pinning policy that
always works.

Refs #2498

Change-Id: I2a294aee460947cd3aad5e23869cead1b99fd610