[openmp][AMDGPU] - Correct getNumberOfBlocks calculation.

This patch fixes the 6 amdgpu buildbot lit test failures
introduced by https://reviews.llvm.org/D135444.
      libomptarget :: amdgcn-amd-amdhsa :: mapping/reduction_implicit_map.cpp
      libomptarget :: amdgcn-amd-amdhsa :: offloading/cuda_no_devices.c
      libomptarget :: amdgcn-amd-amdhsa :: offloading/target-teams-atomic.c
      libomptarget :: amdgcn-amd-amdhsa-LTO :: mapping/reduction_implicit_map.cpp
      libomptarget :: amdgcn-amd-amdhsa-LTO :: offloading/cuda_no_devices.c
      libomptarget :: amdgcn-amd-amdhsa-LTO :: offloading/target-teams-atomic.c

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D137261