linuxkpi: Add mmu_context.h
Provide dummy functions needed for drm-kmod.
Reviewed by: bz
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D35566
(cherry picked from commit a95f28800a0169d7cfbfb9aabc5ae9410c20ecd1)