drm/nouveau: fix dma_address check for CPU/GPU sync

AGP for example doesn't have a dma_address array.
Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210614110517.1624-1-christian.koenig@amd.com

drivers/gpu/drm/nouveau/nouveau_bo.c

@@ -546,7 +546,7 @@ nouveau_bo_sync_for_device(struct nouveau_bo *nvbo)
 	struct ttm_tt *ttm_dma = (struct ttm_tt *)nvbo->bo.ttm;
 	int i, j;
 
-	if (!ttm_dma)
+	if (!ttm_dma || !ttm_dma->dma_address)
 		return;
 	if (!ttm_dma->pages) {
 		NV_DEBUG(drm, "ttm_dma 0x%p: pages NULL\n", ttm_dma);
@@ -582,7 +582,7 @@ nouveau_bo_sync_for_cpu(struct nouveau_bo *nvbo)
 	struct ttm_tt *ttm_dma = (struct ttm_tt *)nvbo->bo.ttm;
 	int i, j;
 
-	if (!ttm_dma)
+	if (!ttm_dma || !ttm_dma->dma_address)
 		return;
 	if (!ttm_dma->pages) {
 		NV_DEBUG(drm, "ttm_dma 0x%p: pages NULL\n", ttm_dma);