linuxkpi: Add <video/of_display_timing.h> to dummy headers
[Why]
This header is being used by DRM drivers in Linux 5.18.
Reviewed by: emaste, manu
Approved by: emaste, manu
Differential Revision: https://reviews.freebsd.org/D43028
(cherry picked from commit b30637c100f1c0d03b2155481744a17da3d955ce)