Fix PME checks in GPU update assignment decision

Fix PME checks in GPU update assignment decision

The decideWhetherToUseGpuForUpdate() function performed a check on
whether PME is on GPU, but neglected to check if PME is actually
active. Therefore, cases without PME (e.g. reaction field) were
unintentionally being prevented from running update on GPU. This change
adds a check whether pmeRunMode != PmeRunMode::None

Change-Id: Ic49f99b2e2afddabd7e2c8e68ab3d6d9325dc07a