amd64 pmap: Tidy up pmap_promote_pde() calls

Since pmap_ps_enabled() is true by default, check it inside of
pmap_promote_pde() instead of at every call site.

Modify pmap_promote_pde() to return true if the promotion succeeded and
false otherwise. Use this return value in a couple places.

Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D40744