kernel - Improve pmap_change_attr() when used on the DMAP

kernel - Improve pmap_change_attr() when used on the DMAP

* When used on the DMAP's 1GB or 2MB pages pmap_change_attr()
  basically didn't work because it tries to access terminal PTEs
  (and there aren't any), instead accidently accessing the
  underlying physical memory and causing corruption.

* Fix this by (1) Forcing the DMAP to use 2MB pages and
  (2) special-casing DMAP addresses in pmap_change_attr()
  to act on the PDEs.

* This is not a complete fix because the granularity for
  any DMAP VAs will still be ~2MB and not 4K.  Its use by
  the DRM subsystem requires more investigation.  But the
  changes should improve some of the reported GPU corruption
  issues.

Reported-by: peeter
Testing-by: multiple