qxl: fix null-pointer crash during suspend

"crtc->helper_private" is not initialized by the QXL driver and thus the
"crtc_funcs->disable" call would crash (resulting in suspend failure).
Fix this by converting the suspend/resume functions to use the
drm_mode_config_helper_* helpers.

Tested system sleep with QEMU 3.0 using "echo mem > /sys/power/state".
During suspend the following message is visible from QEMU:

    spice/server/display-channel.c:2425:display_channel_validate_surface: canvas address is 0x7fd05da68308 for 0 (and is NULL)
    spice/server/display-channel.c:2426:display_channel_validate_surface: failed on 0

This seems to be triggered by QXL_IO_NOTIFY_CMD after
QXL_IO_DESTROY_PRIMARY_ASYNC, but aside from the warning things still
seem to work (tested with both the GTK and -spice options).
Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Link: http://patchwork.freedesktop.org/patch/msgid/20180904202747.14968-1-peter@lekensteyn.nlSigned-off-by: Gerd Hoffmann <kraxel@redhat.com>

drivers/gpu/drm/qxl/qxl_drv.c

@@ -136,20 +136,11 @@ static int qxl_drm_freeze(struct drm_device *dev)
 {
 	struct pci_dev *pdev = dev->pdev;
 	struct qxl_device *qdev = dev->dev_private;
-	struct drm_crtc *crtc;
-
-	drm_kms_helper_poll_disable(dev);
-
-	console_lock();
-	qxl_fbdev_set_suspend(qdev, 1);
-	console_unlock();
+	int ret;
 
-	/* unpin the front buffers */
-	list_for_each_entry(crtc, &dev->mode_config.crtc_list, head) {
-		const struct drm_crtc_helper_funcs *crtc_funcs = crtc->helper_private;
-		if (crtc->enabled)
-			(*crtc_funcs->disable)(crtc);
-	}
+	ret = drm_mode_config_helper_suspend(dev);
+	if (ret)
+		return ret;
 
 	qxl_destroy_monitors_object(qdev);
 	qxl_surf_evict(qdev);
@@ -175,14 +166,7 @@ static int qxl_drm_resume(struct drm_device *dev, bool thaw)
 	}
 
 	qxl_create_monitors_object(qdev);
-	drm_helper_resume_force_mode(dev);
-
-	console_lock();
-	qxl_fbdev_set_suspend(qdev, 0);
-	console_unlock();
-
-	drm_kms_helper_poll_enable(dev);
-	return 0;
+	return drm_mode_config_helper_resume(dev);
 }
 
 static int qxl_pm_suspend(struct device *dev)