Try to catch a bug we've seen in the wild where the shrinker purges the pd/pdp from under us while allocating our paging structures. References: https://bugs.freedesktop.org/show_bug.cgi?id=104773 Signed-off-by: Matthew Auld <matthew.auld@intel.com> Cc: Chris Wilson <chris@chris-wilson.co.uk> Link: https://patchwork.freedesktop.org/patch/msgid/20180131191453.12676-1-matthew.auld@intel.com Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Link: https://patchwork.freedesktop.org/patch/msgid/20180131214440.7141-2-chris@chris-wilson.co.uk