- agp_generic_unbind_memory: flush AGP TLB before unwiring pages
- agp_bind_pages: assert that pages have been wired down

MFC after:	1 month


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273965 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f