drm/ttm: fix missing NULL check in the new page pool

The pool parameter can be NULL if we free through the shrinker.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Tested-by: Martin Peres <martin.peres@mupuf.org>
Acked-by: Martin Peres <martin.peres@mupuf.org>
Reported-by: Andy Lavr <andy.lavr@gmail.com>
Tested-by: Andy Lavr <andy.lavr@gmail.com>
Link: https://patchwork.freedesktop.org/patch/399365/