drm/doc: Move hdmi infoframe docs

.. next to all the other sink helpers. The rect library is more used
for handling plane clipping, so belongs to those imo.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190130163006.28945-1-daniel.vetter@ffwll.ch