Fixed two PME issues with MPI+OpenMP

Change 272736bc partially fixed #1388, but broke the more general
case of multiple MPI communication pulses in PME. Change 272736bc
incorrectly changed tx1 and ty1. This change has been reverted.

Change 27189bba fixed the incorrect PME grid reduction with multiple
thread grid overlap in y. But it broke the, much more common, case
where the y-size of the PME grid is not divisible by the domains in y.
This change, incorrectly, changed buf_my.

Now buf_my is set to the correct value, which solves both issues.

Fixes #1578.
Refs #1388 and #1572.

Change-Id: Id2d7d013a3b8cdc04eda1fb026567088a38ec81f