LinuxKPI: Import MTRR support functions from drm-kmod

They are superseded by PAT and mostly useless nowadays but still can be
used on Pentium III/IV era processors. Unlike drm-kmod version, this one
ignores MTRR if PAT is available that fixes confusing "Failed to add WC
MTRR for [0xXXXX-0xYYYY]: 22; performance may suffer" message often
appearing during drm-kmod initialization process.

MFC after: 1 week
Reviewed by: hselasky, manu
Differential Revision: https://reviews.freebsd.org/D33561