drm/amdgpu: add some VM PD/PT iterators v2

Both a leaf as well as dfs iterator to walk over all the PDs/PTs.

v2: update comments and fix for_each_amdgpu_vm_pt_dfs_safe

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>