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=35572Reviewed-by: NEugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_i2c.c

@@ -390,7 +390,7 @@ int intel_setup_gmbus(struct drm_device *dev)
 		bus->has_gpio = intel_gpio_setup(bus, i);
 
 		/* XXX force bit banging until GMBUS is fully debugged */
-		if (bus->has_gpio)
+		if (bus->has_gpio && IS_GEN2(dev))
 			bus->force_bit = true;
 	}