amd64: cleanup pmap_init_pat().

The pmap_works variable is always true for amd64. Remove it, the
branch in the initialization taken when false, and corresponding
sysctl.

Remove pat_table[] local array, work on pat_index[] directly.

Collapse whole initialization to not override already assigned values.

Add comment explaining the choice for PAT4 and PAT7.

Reviewed by: alc, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
MFC note: Leave the sysctl around
Differential revision: https://reviews.freebsd.org/D19225