drm/ttm: allow mapping BOs while they are still on the swap list

With shared reservation objects it is possible that we want to temporary
kmap an BO while it is still on the swap list.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>