[openmp][amdgpu] Tear down amdgpu plugin accurately

Moves DeviceInfo global to heap to accurately control lifetime.
Moves calls from libomptarget to deinit_plugin later, plugins need to stay
alive until very shortly before libomptarget is destructed.

Leaving the deinit_plugin calls where initially inserted hits use after
free from the dynamic_module.c offloading test (verified with valgrind
 that the new location is sound with respect to this)

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D130714