drm/nouveau: Use write-combined maps for coherent

On Tegra devices, uncached maps traslate to device memory, causing
unaligned accesses by userspace resulting in a SIGBUS. Instead, use
write-combined maps to ensure proper access.

It was also determined that discrete cards should also use
write-combined maps for coherent. Thus this change is made for all gpu
types.

Signed-off-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Co-developed-by: Aaron Kling <webgeek1234@gmail.com>
Signed-off-by: Aaron Kling <webgeek1234@gmail.com>

drivers/gpu/drm/nouveau/nouveau_sgdma.c

@@ -72,9 +72,7 @@ nouveau_sgdma_create_ttm(struct ttm_buffer_object *bo, uint32_t page_flags)
 	struct nouveau_sgdma_be *nvbe;
 	enum ttm_caching caching;
 
-	if (nvbo->force_coherent)
-		caching = ttm_uncached;
-	else if (drm->agp.bridge)
+	if (nvbo->force_coherent || drm->agp.bridge)
 		caching = ttm_write_combined;
 	else
 		caching = ttm_cached;