drm/ttm: make the pool shrinker lock a mutex

set_pages_wb() might sleep and so we can't do this in an atomic context.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reported-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Tested-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Fixes: d099fc8f ("drm/ttm: new TT backend allocation pool v3")
Reviewed-by: Huang Rui <ray.huang@amd.com>
Link: https://patchwork.freedesktop.org/patch/413409/