drm/i915/skl: Prevent unclaimed register writes on skylake.

I'm getting unclaimed register writes when checking the WM registers
after the crtc is disabled. So I would imagine those are guarded by
the crtc power well. Fix this by not reading out wm state when the
power well is off.

Cc: stable@vger.kernel.org # v4.3
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92181Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_pm.c

@@ -2818,7 +2818,12 @@ void skl_ddb_get_hw_state(struct drm_i915_private *dev_priv,
 	int plane;
 	u32 val;
 
+	memset(ddb, 0, sizeof(*ddb));
+
 	for_each_pipe(dev_priv, pipe) {
+		if (!intel_display_power_is_enabled(dev_priv, POWER_DOMAIN_PIPE(pipe)))
+			continue;
+
 		for_each_plane(dev_priv, pipe, plane) {
 			val = I915_READ(PLANE_BUF_CFG(pipe, plane));
 			skl_ddb_entry_init_from_hw(&ddb->plane[pipe][plane],