drm/display: hdmi: Mention Infoframes testing with edid-decode

drm/display: hdmi: Mention Infoframes testing with edid-decode

edid-decode gained recently support to check that infoframes are
compliant and match the EDID the monitor exposes.

Since the HDMI helpers provide those infoframes in debugfs, it makes it
easy to check from userspace that the drivers (and helpers) behave
properly.

Let's document it.

Cc: Hans Verkuil <hverkuil@xs4all.nl>
Reviewed-by: Hans Verkuil <hverkuil@xs4all.nl>
Link: https://patchwork.freedesktop.org/patch/msgid/20250312-drm-hdmi-state-docs-v2-2-6352a5d68d5b@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>