drm/ttm: make sure pool pages are cleared

The old implementation wasn't consistend on this.

But it looks like we depend on this so better bring it back.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reported-and-tested-by: Mike Galbraith <efault@gmx.de>
Fixes: d099fc8f

 ("drm/ttm: new TT backend allocation pool v3")
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210210160549.1462-1-christian.koenig@amd.com