drm/atomic: Convert atomic 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/20170912133749.6532-3-maarten.lankhorst@linux.intel.com
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>

drivers/gpu/drm/drm_atomic.c

@@ -2231,7 +2231,7 @@ int drm_mode_atomic_ioctl(struct drm_device *dev,
 			(arg->flags & DRM_MODE_PAGE_FLIP_EVENT))
 		return -EINVAL;
 
-	drm_modeset_acquire_init(&ctx, 0);
+	drm_modeset_acquire_init(&ctx, DRM_MODESET_ACQUIRE_INTERRUPTIBLE);
 
 	state = drm_atomic_state_alloc(dev);
 	if (!state)
@@ -2342,8 +2342,9 @@ out:
 
 	if (ret == -EDEADLK) {
 		drm_atomic_state_clear(state);
-		drm_modeset_backoff(&ctx);
-		goto retry;
+		ret = drm_modeset_backoff(&ctx);
+		if (!ret)
+			goto retry;
 	}
 
 	drm_atomic_state_put(state);