drm/i915/gen9: Initialize intel_state->active_crtcs during WM sanitization (v2)

intel_state->active_crtcs is usually only initialized when doing a
modeset.  During our first atomic commit after boot, we're effectively
faking a modeset to sanitize the DDB/wm setup, so ensure that this field
gets initialized before use.

v2:
 - Don't clobber active_crtcs if our first commit really is a modeset
   (Maarten)
 - Grab connection_mutex when faking a modeset during sanitization
   (Maarten)

Reported-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Tested-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466196140-16336-2-git-send-email-matthew.d.roper@intel.com
Cc: stable@vger.kernel.org #v4.7+

drivers/gpu/drm/i915/intel_pm.c

@@ -3920,9 +3920,24 @@ skl_compute_ddb(struct drm_atomic_state *state)
 	 * pretend that all pipes switched active status so that we'll
 	 * ensure a full DDB recompute.
 	 */
-	if (dev_priv->wm.distrust_bios_wm)
+	if (dev_priv->wm.distrust_bios_wm) {
+		ret = drm_modeset_lock(&dev->mode_config.connection_mutex,
+				       state->acquire_ctx);
+		if (ret)
+			return ret;
+
 		intel_state->active_pipe_changes = ~0;
 
+		/*
+		 * We usually only initialize intel_state->active_crtcs if we
+		 * we're doing a modeset; make sure this field is always
+		 * initialized during the sanitization process that happens
+		 * on the first commit too.
+		 */
+		if (!intel_state->modeset)
+			intel_state->active_crtcs = dev_priv->active_crtcs;
+	}
+
 	/*
 	 * If the modeset changes which CRTC's are active, we need to
 	 * recompute the DDB allocation for *all* active pipes, even