drm/amdgpu: new resource cursor (v2)

Allows to walk over the drm_mm nodes in a TTM resource object.

v2: squash in fix from Felix

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>