Fix twin-cutoff for GPU LJ pot-switch

Fix twin-cutoff for GPU LJ pot-switch

With rcoulomb>rvdw, as used with PME tuning, the GPU kernels performed
the LJ cut-off check before applying the potential-switch to the LJ
forces and energies.

Fixes #2007.

Change-Id: Ie2f6fdfec10bdb8800123e6fa299839a31bc0f93