drm/amdgpu: use the new cursor in the VM code

Separate the drm_mm_node walking from the actual handling.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Oak Zeng <Oak.Zeng@amd.com>
Tested-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Arunpravin <Arunpravin.PaneerSelvam@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>