Conditionally pin GPU-related grid data

Conditionally pin GPU-related grid data

Data that is transferred to the GPU when the buffer ops is offloaded is
now only pinned when the nonbonded module uses GPU offload avoidign the
runtime errors encountered when a GPU-enabled build does not detect a
GPU and therefore the CUDA runtime refuses to register the memory.

Refs #2817 #2934

Change-Id: Iabbc0d9f37fad0e88cd39a078af1346e8f713ec1