drm/ttm: Fix accounting error when fail to get pages for pool

When fail to get needed page for pool, need to put allocated pages
into pool. But current code has a miscalculation of allocated pages,
correct it.

Signed-off-by: Xiangliang.Yu <Xiangliang.Yu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Monk Liu <monk.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org