free energy with perturbed charges is not correct with PME GPU mixed mode

The nbnxm-free-energy and nbnxm-free-energy-vv tests fail with mixed mode PME offload (-pme gpu -pmefft cpu). This has been reproduced with master, 2021 release, as far back as the original change that introduced the FEP PME GPU (!15 (merged) using f19e4c64).

comparing energy file /nethome/pszilard-projects/gromacs/_rtest_/regressiontests-master_gpu37/complex/nbnxn-free-energy/reference_s.edr and ener.edr

There are 43 terms in the energy files

There are 8 terms to compare in the energy files

Coul. recip.     step   0:       175.572,  step   0:      184.227
Coul. recip.     step   4:       180.383,  step   4:      186.665
Coul. recip.     step   8:       185.225,  step   8:      189.669
Coul. recip.     step  12:       189.819,  step  12:      192.743
Coul. recip.     step  16:       193.965,  step  16:      195.469