drm: review locking for drm_fb_helper_restore_fbdev_mode

... it's required. Fix up exynos and the cma helper, and add a
corresponding WARN_ON to drm_fb_helper_restore_fbdev_mode.

Note that tegra calls the fbdev cma helper restore function also from
it's driver-load callback. Which is a bit against current practice,
since usually the call is only from ->lastclose, and initial setup is
done by drm_fb_helper_initial_config.

Also add the relevant drm DocBook entry.

v2: Add promised WARN to restore_fbdev_mode.

Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/exynos/exynos_drm_fbdev.c

@@ -376,5 +376,7 @@ void exynos_drm_fbdev_restore_mode(struct drm_device *dev)
 	if (!private || !private->fb_helper)
 		return;
 
+	drm_modeset_lock_all(dev);
 	drm_fb_helper_restore_fbdev_mode(private->fb_helper);
+	drm_modeset_unlock_all(dev);
 }