LinuxKPI: upstream a collection of drm-kmod conflicting changes

The upcoming in-kernel implementations for LinuxKPI based on work on
iwlwifi (and other wireless drivers) conflicts in a few places with
the drm-kmod graphics work outside the base system.

In order to transition smoothly extract the conflicting bits.
This included "unaligned" accessor functions, sg_pcopy_from_buffer(),
IS_*() macros (to be further restricted in the future), power management
bits (possibly no longer conflicting with DRM), and other minor changes.

Obtained-from: bz_iwlwifi
Sponsored-by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: kib, hselasky, manu, bdragon (looked at earlier versions)
Differential Revision: https://reviews.freebsd.org/D26598

(cherry picked from commit 4abbf816bf06aa70200c5d1d976dd61c2752bdaf)