Disable GPU update/constraints when neither PME nor buffer ops are offloaded.

Disable GPU update/constraints when neither PME nor buffer ops are offloaded.

Using the GPU-version of update makes sense if forces are already on the GPU,
i.e. if at least:
1. PME is on the GPU (there should be a copy of coordinates on a GPU in rvec
   format for PME spread).
2. Non-bonded interactions and buffer ops are on the GPU.

This is temporary solution, needed because the buffer ops offload switch is
operated by the environment variable. More favorable behavior would be to
switch on the GPU buffer ops in the second case rather then disabling the
GPU update.

Refs. #2816.

Change-Id: I37a9969dd6c74dcfa41a95da13ae54d014c9ea60