agp: fix scratch page cleanup

In error cleanup of agp_backend_initialize() and in agp_backend_cleanup(),
agp_destroy_page() is passed virtual address of the scratch page. This
leads to a kernel warning if the initialization fails (or upon regular
cleanup) as pointer to struct page should be passed instead.

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: Dave Airlie <airlied@redhat.com>