amd64 pmap: convert panics to KASSERTs

With the recent changes to pmap_demote_DMAP(), the calls to
pmap_demote_pdpe() and pmap_demote_pde_mpte() should never fail, so
we change the associated panics to KASSERTs.

(cherry picked from commit 6a7761b4d27c99b3b548f2d948b88bf1430ee636)