drm/i915: remove unused DDC PIN macros

Their user has vanished in the course of history. Remove.

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181115105237.1237-2-jani.nikula@intel.com