amd64 pmap: Tidy up pmap_promote_pde() calls

[alc]

Summary

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.

[markj]

Is there any reason not to make the same change on arm64? (Or riscv for that matter.)

[alc]

In D40744#927052, @markj wrote:

Is there any reason not to make the same change on arm64? (Or riscv for that matter.)

In principle, no. I will look at arm64 soon. The pmap->pm_stage == PG_STAGE1 checks on arm64 complicate things.