drm/atomic: Convert pageflip ioctl locking to interruptible.

Pass DRM_MODESET_ACQUIRE_INTERRUPTIBLE to acquire_init, and handle
drm_modeset_backoff which can now fail by returning the error.
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170912133749.6532-6-maarten.lankhorst@linux.intel.comReviewed-by: Emil Velikov <emil.l.velikov@gmail.com>

drivers/gpu/drm/drm_plane.c

@@ -986,7 +986,7 @@ int drm_mode_page_flip_ioctl(struct drm_device *dev,
 		return -EINVAL;
 	}
 
-	drm_modeset_acquire_init(&ctx, 0);
+	drm_modeset_acquire_init(&ctx, DRM_MODESET_ACQUIRE_INTERRUPTIBLE);
 retry:
 	ret = drm_modeset_lock(&crtc->mutex, &ctx);
 	if (ret)
@@ -1075,7 +1075,8 @@ int drm_mode_page_flip_ioctl(struct drm_device *dev,
 	crtc->primary->old_fb = NULL;
 
 	if (ret == -EDEADLK) {
-		drm_modeset_backoff(&ctx);
+		ret = drm_modeset_backoff(&ctx);
+		if (!ret)
 			goto retry;
 	}