drm/i915: add bxt gmbus support

For BXT gmbus is pulled from PCH to CPU. From implementation point of
view only pin pair configuration will change. The existing
implementation supports all platforms previous to GEN8 and also SKL. But
for BXT pin pair configuration is completely different than SKL or other
previous GEN's. This patch introduces the new pin pair configuration
structure specific to BXT and also ensures every real gmbus port has a
gpio pin.

v3 by Jani: with the platform independent prep work in place, the bxt
enabling reduces to a fairly trivial patch. Credits are due Sunil for
giving me the ideas (with his patches) what the platform independent
parts should look like.

v4: Fix intel_hdmi_init_connector() for bxt. Abstract gmbus_pin access
more. s/GPU/PCH/ in commit message.

v5: Rebase.

Issue: VIZ-3574
Signed-off-by: A.Sunil Kamath <sunil.kamath@intel.com>
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>