drm/i915: reenable gmbus on gen3+ again

With the rework to merge the bit-banging fallback into the gmbus
i2c adapter we've gotten rid of the deadlock possibility that
originally lead to the disabling of this code.

This reverts the revert

commit 826c7e41


Author: Jean Delvare <khali@linux-fr.org>
Date:   Sat Jun 4 19:34:56 2011 +0000

    Revert "drm/i915: Enable GMBUS for post-gen2 chipsets"

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=35572
Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>