drm/i915: rename GMBUS_PORT_* macros as GMBUS_PIN_*

The specs refer to pin pairs. Start moving towards using pin rather than
port all around to avoid confusion. No functional changes.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>