drm/i915/bxt: Disable power well support

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

        commit 9f836f90
        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.htmlSigned-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.comReviewed-by: Imre Deak <imre.deak@intel.com>

drivers/gpu/drm/i915/intel_runtime_pm.c

@@ -1911,6 +1911,11 @@ sanitize_disable_power_well_option(const struct drm_i915_private *dev_priv,
 	if (disable_power_well >= 0)
 		return !!disable_power_well;
 
+	if (IS_BROXTON(dev_priv)) {
+		DRM_DEBUG_KMS("Disabling display power well support\n");
+		return 0;
+	}
+
 	return 1;
 }