drm/ast: Fix double lock at PM resume

The recent commit [3ea87855: drm/helper: lock all around force mode
restore] introduced drm_modeset_lock_all() in
drm_helper_resume_force_mode() itself, while ast driver still takes
this lock before calling it.  Remove the caller side lock for avoid a
fatal deadlock.
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/ast/ast_drv.c

@@ -94,9 +94,7 @@ static int ast_drm_thaw(struct drm_device *dev)
 	ast_post_gpu(dev);
 
 	drm_mode_config_reset(dev);
-	drm_modeset_lock_all(dev);
 	drm_helper_resume_force_mode(dev);
-	drm_modeset_unlock_all(dev);
 
 	console_lock();
 	ast_fbdev_set_suspend(dev, 0);