linuxkpi: Add hdmi helpers

This is a direct port of the Linux code as the licence allows it, so
style(9) isn't respected to allow applying directly the upstream commits.
Do not add it to linuxkpi directly but add a new linuxkpi_hdmi module
that drm modules will require later, no need to bloat linuxkpi more.

Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D39122