drm/ttm: Choose a pool to shrink correctly in ttm_dma_pool_shrink_scan().

drm/ttm: Choose a pool to shrink correctly in ttm_dma_pool_shrink_scan().

We can use "unsigned int" instead of "atomic_t" by updating start_pool
variable under _manager->lock. This patch will make it possible to avoid
skipping when choosing a pool to shrink in round-robin style, after next
patch changes mutex_lock(_manager->lock) to !mutex_trylock(_manager->lork).

Signed-off-by: Tetsuo Handa <[email protected]>
Cc: stable <[email protected]> [3.3+]
Signed-off-by: Dave Airlie <[email protected]>