Avoid seg fault when PME-PP GPU comm flag is used in isolation

Avoid seg fault when PME-PP GPU comm flag is used in isolation

Improves stability by ensuring that GPU buffer ops flag is
automatically set if GPU PME-PP flag is set (in the same way as was
already done for GPU DD flag). Otherwise get a seg fault if PME-PP GPU
env trigger is used without either GPU DD or GPU Buffer ops env
triggers).