drm: Introduce drm_fb_helper_prepare()

To implement hotplug detection in a race-free manner, drivers must call
drm_kms_helper_poll_init() before hotplug events can be triggered. Such
events can be triggered right after any of the encoders or connectors
are initialized. At the same time, if the drm_fb_helper_hotplug_event()
helper is used by a driver, then the poll helper requires some parts of
the FB helper to be initialized to prevent a crash.

At the same time, drm_fb_helper_init() requires information that is not
necessarily available at such an early stage (number of CRTCs and
connectors), so it cannot be used yet.

Add a new helper, drm_fb_helper_prepare(), that initializes the bare
minimum needed to allow drm_kms_helper_poll_init() to execute and any
subsequent hotplug events to be processed properly.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/qxl/qxl_fb.c

@@ -676,9 +676,12 @@ int qxl_fbdev_init(struct qxl_device *qdev)
 
 	qfbdev->qdev = qdev;
 	qdev->mode_info.qfbdev = qfbdev;
-	qfbdev->helper.funcs = &qxl_fb_helper_funcs;
 	spin_lock_init(&qfbdev->delayed_ops_lock);
 	INIT_LIST_HEAD(&qfbdev->delayed_ops);
+
+	drm_fb_helper_prepare(qdev->ddev, &qfbdev->helper,
+			      &qxl_fb_helper_funcs);
+
 	ret = drm_fb_helper_init(qdev->ddev, &qfbdev->helper,
 				 qxl_num_crtc /* num_crtc - QXL supports just 1 */,
 				 QXLFB_CONN_LIMIT);