drm/i915: Disable decoupled MMIO

The decoupled MMIO feature doesn't work as intended by HW team. Enabling
it with forcewake will only make debugging efforts more difficult, so
let's disable it.

Fixes: 85ee17eb ("drm/i915/bxt: Broxton decoupled MMIO")
Cc: Zhe Wang <zhe1.wang@intel.com>
Cc: Praveen Paneri <praveen.paneri@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v4.10+
Signed-off-by: Kai Chen <kai.chen@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170523215812.18328-2-kai.chen@intel.com


(cherry picked from commit 0051c10a)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>