drm/ttm: optimize the pool shrinker a bit v2

Switch back to using a spinlock again by moving the IOMMU unmap outside
of the locked region.

This avoids contention especially while freeing pages.

v2: Add a comment explaining why we need sync_shrinkers().

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210820120528.81114-3-christian.koenig@amd.com