drm/fourcc: Document open source user waiver

It's a bit a FAQ, and we really can't claim to be the authoritative
source for allocating these numbers used in many standard extensions
if we tell closed source or vendor stacks in general to go away.

Iirc this was already clarified in some vulkan discussions, but I
can't find that anywhere anymore. At least not in a public link.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Alex Deucher <alexdeucher@gmail.com>
Cc: Daniel Stone <daniel@fooishbar.org>
Cc: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: Neil Trevett <ntrevett@nvidia.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Acked-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: David Airlie <airlied@gmail.com>
Acked-by: Marek Olšák <maraeo@gmail.com>
Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Acked-by: Jason Ekstrand <jason.ekstrand@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221123192437.1065826-1-daniel.vetter@ffwll.ch