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>
diff --git a/include/drm/drm_crtc.h b/include/drm/drm_crtc.h
index e90c8dc..8b77627 100644
--- a/include/drm/drm_crtc.h
+++ b/include/drm/drm_crtc.h
@@ -867,6 +867,7 @@
 
 extern void drm_modeset_lock_all(struct drm_device *dev);
 extern void drm_modeset_unlock_all(struct drm_device *dev);
+extern void drm_warn_on_modeset_not_all_locked(struct drm_device *dev);
 
 extern int drm_crtc_init(struct drm_device *dev,
 			 struct drm_crtc *crtc,