drm/ttm: fix re-init of global structures

When a driver unloads without unloading TTM we don't correctly
clear the global structures leading to errors on re-init.

Next step should probably be to remove the global structures and
kobjs all together, but this is tricky since we need to maintain
backward compatibility.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Tested-by: Karol Herbst <kherbst@redhat.com>
CC: stable@vger.kernel.org # 5.0.x
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>