drm/nouveau/bo: consider DMA buffers on x86 only

The DMA API has different semantics on different architectures.
Currently on arm64, it can only provide memory from a small pool which
dries up quickly if we attempt to allocate big buffers from it.

Do not consider that option when running on non-x86, since regular TTM
buffers are the (current) best-fit for ARM platforms.
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_bo.c

@@ -1502,7 +1502,7 @@ nouveau_ttm_tt_populate(struct ttm_tt *ttm)
 	}
 #endif
 
-#ifdef CONFIG_SWIOTLB
+#if IS_ENABLED(CONFIG_SWIOTLB) && IS_ENABLED(CONFIG_X86)
 	if (swiotlb_nr_tbl()) {
 		return ttm_dma_populate((void *)ttm, dev->dev);
 	}
@@ -1570,7 +1570,7 @@ nouveau_ttm_tt_unpopulate(struct ttm_tt *ttm)
 	}
 #endif
 
-#ifdef CONFIG_SWIOTLB
+#if IS_ENABLED(CONFIG_SWIOTLB) && IS_ENABLED(CONFIG_X86)
 	if (swiotlb_nr_tbl()) {
 		ttm_dma_unpopulate((void *)ttm, dev->dev);
 		return;