drm/dp-helper: Deprecate old i2c-over-dp_aux heleprs

Only gma500 is still using this, once that's converted we can kill all
this code. If that conversion doesn't happen soonish I think we should
just move this helper code into the gma500 driver itself to avoid
abuse from new drivers.

Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Cc: Alan Cox <alan@linux.intel.com>
Cc: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>