drm/i915/bxt: Disable power well support

drm/i915/bxt: Disable power well support

BXT power well support is not yet stable.  Starting with patch

        commit 9f836f9016ad5320e0c9230419d2102cf15a28aa
        Author: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
        Date:   Mon Nov 16 16:20:01 2015 +0100

            drm/i915/gen9: Turn DC handling into a power well

DPMS off operations may actually cause the entire system to powerdown or
reboot.  Disable power well support for now until Broxton gets fixes
similar to what we have for SKL.

Cc: Imre Deak <imre.deak@intel.com>
Cc: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
References: http://lists.freedesktop.org/archives/intel-gfx/2015-November/081037.html
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448990818-11005-1-git-send-email-matthew.d.roper@intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>