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.

Reviewed by: kib, markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D51180