arm64 pmap: Tidy up pmap_promote_l2() calls

Since pmap_ps_enabled() is true by default, check it inside of
pmap_promote_l2() instead of at every call site.

Modify pmap_promote_l2() to return true if the promotion succeeded and
false otherwise.

(A similar change was applied to the amd64 pmap in 0d2f98c2f092.)

Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D40781