drm/display: hdmi: Create documentation section

drm/display: hdmi: Create documentation section

We have had documentation for the public functions in the HDMI helpers,
but those were never referenced anywhere and thus not compiled as part
of the doc.

Let's add a section.

Reviewed-by: Dmitry Baryshkov <lumag@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250312-drm-hdmi-state-docs-v2-1-6352a5d68d5b@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>