drm/i915: Fix locking in DRRS debugfs

Grab the crtc mutex so that looking at the crtc state is
actually safe.
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221003113249.16213-5-ville.syrjala@linux.intel.com

drivers/gpu/drm/i915/display/intel_drrs.c

@@ -301,8 +301,14 @@ void intel_crtc_drrs_init(struct intel_crtc *crtc)
 static int intel_drrs_debugfs_status_show(struct seq_file *m, void *unused)
 {
 	struct intel_crtc *crtc = m->private;
-	const struct intel_crtc_state *crtc_state =
-		to_intel_crtc_state(crtc->base.state);
+	const struct intel_crtc_state *crtc_state;
+	int ret;
+
+	ret = drm_modeset_lock_single_interruptible(&crtc->base.mutex);
+	if (ret)
+		return ret;
+
+	crtc_state = to_intel_crtc_state(crtc->base.state);
 
 	mutex_lock(&crtc->drrs.mutex);
 
@@ -321,6 +327,8 @@ static int intel_drrs_debugfs_status_show(struct seq_file *m, void *unused)
 
 	mutex_unlock(&crtc->drrs.mutex);
 
+	drm_modeset_unlock(&crtc->base.mutex);
+
 	return 0;
 }