drm/i915: Use the correct GMCH_CTRL register for Sandybridge+

The GMCH_CTRL register (or MGCC in the spec) is at a different address
on Sandybridge, and the address to which we currently write to is
undefined. These stray writes appear to upset (hard hang) my Ivybridge
machine whilst it is in UEFI mode.

Note that the register is still marked as locked RO on Sandybridge, so
vgaarb is still dysfunctional.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>