drm/nouveau: Revert "bus: remove cpu_coherent flag"

This reverts commit aff51175cd.

The commit caused fence timeouts within nvc0_screen_destroy and most likely
other places as well.

The most obvious effect is, that userspace processes take minutes to
actually quit.

Signed-off-by: Karol Herbst <karolherbst@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nvkm/engine/device/pci.c

@@ -1614,6 +1614,7 @@ nvkm_device_pci_func = {
 	.fini = nvkm_device_pci_fini,
 	.resource_addr = nvkm_device_pci_resource_addr,
 	.resource_size = nvkm_device_pci_resource_size,
+	.cpu_coherent = !IS_ENABLED(CONFIG_ARM),
 };
 
 int

drivers/gpu/drm/nouveau/nvkm/engine/device/tegra.c

@@ -245,6 +245,7 @@ nvkm_device_tegra_func = {
 	.fini = nvkm_device_tegra_fini,
 	.resource_addr = nvkm_device_tegra_resource_addr,
 	.resource_size = nvkm_device_tegra_resource_size,
+	.cpu_coherent = false,
 };
 
 int