video/hdmi: Relicense header under MIT license

OpenBSD wants to reuse this file but needs the license to be more
permissive.

Acked-by: Alban Bedel <alban.bedel@avionic-design.de>
Acked-by: Daniel Vetter <daniel.vetter@intel.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>