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

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

commit 9afae2719273fa1d406829bf3498f82dbdba71c7 upstream.

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>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>