drm/radeon: add extra check in radeon_ttm_tt_unpin_userptr

We somehow try to free the SG table twice.

Bugs: https://bugs.freedesktop.org/show_bug.cgi?id=89734

Signed-off-by: Christian König <christian.koenig@amd.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_ttm.c

@@ -598,6 +598,10 @@ static void radeon_ttm_tt_unpin_userptr(struct ttm_tt *ttm)
 	enum dma_data_direction direction = write ?
 		DMA_BIDIRECTIONAL : DMA_TO_DEVICE;
 
+	/* double check that we don't free the table twice */
+	if (!ttm->sg->sgl)
+		return;
+
 	/* free the sg table and pages again */
 	dma_unmap_sg(rdev->dev, ttm->sg->sgl, ttm->sg->nents, direction);