drm/ttm: fix ttm_bo_wait

reservation_object_wait_timeout_rcu() should enable signaling even with a
zero timeout, but ttm_bo_wait() can also be called from atomic context and
then it is not a good idea to do this.
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
  [sumits: fix checkpatch warnings]
Link: http://patchwork.freedesktop.org/patch/msgid/1478553376-18575-3-git-send-email-alexander.deucher@amd.com

drivers/gpu/drm/ttm/ttm_bo.c

@@ -1611,7 +1611,14 @@ EXPORT_SYMBOL(ttm_bo_unmap_virtual);
 int ttm_bo_wait(struct ttm_buffer_object *bo,
 		bool interruptible, bool no_wait)
 {
-	long timeout = no_wait ? 0 : 15 * HZ;
+	long timeout = 15 * HZ;
+
+	if (no_wait) {
+		if (reservation_object_test_signaled_rcu(bo->resv, true))
+			return 0;
+		else
+			return -EBUSY;
+	}
 
 	timeout = reservation_object_wait_timeout_rcu(bo->resv, true,
 						      interruptible, timeout);