drm/ttm: Don't clear page metadata of imported sg pages

These page pointers shouldn't be visible to TTM in the first place, but
until we fix that up, don't clear the page metadata because that
will upset the exporter.

Reported-and-tested-by: Cristoph Haag <haagch.christoph@googleemail.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>