drm/i915: don't register invalid gmbus pins for bdw

Do not expose invalid gmbus pins as i2c devices to userspace.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>