[mlir][rocdl] Change the translation of `GridDim*Op` to `__ockl_get_num_groups`

Currently, `ROCDL::GridDim*Op` is being translated to `__ockl_get_global_size`, however
to match the meaning of `gpu.grid_dim` it should instead be translated to
`__ockl_get_num_groups`. This change would also make it agree with the meaning
of `gridDimx.*` in HIP, see:
https://github.com/ROCm-Developer-Tools/hipamd/blob/develop/include/hip/amd_detail/amd_hip_runtime.h#L257

Difference between the functions:
```
__ockl_get_global_size =  blockDim * numBlocks
__ockl_get_num_groups = numBlocks
```

Reviewed By: krzysz00

Differential Revision: https://reviews.llvm.org/D156009