amd64 pmap: convert some panics to KASSERTs

[alc]

Summary

With the recent changes to pmap_demote_DMAP(), the calls to pmap_demote_pdpe() and pmap_demote_pde_mpte() should never fail.

[kib]

Should the pmap_demote_pde() in pmap_unwire() get the same treatment? As I understand, wire should saved the pt page in radix.

[alc]

In D51180#1168328, @kib wrote:

Should the pmap_demote_pde() in pmap_unwire() get the same treatment? As I understand, wire should saved the pt page in radix.

I think that should be preceded by the changes under discussion in D49442.