drm/ttm: fix dma_fence refcount imbalance on error path

the ttm_bo_add_move_fence takes a reference to the struct dma_fence, but
failed to release it on the error path, leading to a memory leak.
add dma_fence_put before return when error occur.

Signed-off-by: Lin Yi <teroincn@163.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>