drm/ttm: fix kref count mess in ttm_bo_move_to_lru_tail

drm/ttm: fix kref count mess in ttm_bo_move_to_lru_tail

Fixes the following scenario:

1. Page table bo allocated in vram and linked to man->lru.
   tbo->list_kref.refcount=2
2. Page table bo is swapped out and removed from man->lru.
   tbo->list_kref.refcount=1
3. Command submission from userspace.  Page table bo is moved
   to vram.  ttm_bo_move_to_lru_tail() link it to man->lru and
   don't increase the kref count.

Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Flora Cui <Flora.Cui@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org