Revert "drm/i915: reenable gmbus on gen3+ again"

This reverts commit c3dfefa0.

gmbus in 3.4 has simply too many known issues:
- gmbus is too noisy, we need to rework the logging:
  https://bugs.freedesktop.org/show_bug.cgi?id=48248
- zero-length writes cause an OOPS, and they are
  userspace-triggerable:
  https://lkml.org/lkml/2012/3/30/176
- same for zero-length reads:
  https://bugs.freedesktop.org/show_bug.cgi?id=48269



We can try again for 3.5.

Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>