UPSTREAM: drm/i915/intel_i2c: gmbus disabled and reserved ports are invalid

UPSTREAM: drm/i915/intel_i2c: gmbus disabled and reserved ports are invalid

There is no GMBUS "disabled" port 0, nor "reserved" port 7.
For the other 6 ports there is a fixed 1:1 mapping between pin pairs and
gmbus ports, which means every real gmbus port has a gpio pin.

Given these realizations, clean up gmbus initialization.

Tested on Sandybridge (gen 6, PCH == CougarPoint) hardware.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
(cherry picked from commit 2ed06c93a1fce057808894d73167aae03c76deaf)

Change-Id: I82c3398e52b542b024447d4328c914bbf46055dc
Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>