drm/ttm: fix, avoid iomapping system memory

If the memory is not iomem we should not try to
ioremap it. Should fix :

https://bugs.freedesktop.org/show_bug.cgi?id=27822

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Tested-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/ttm/ttm_bo_util.c

@@ -113,7 +113,7 @@ int ttm_mem_reg_ioremap(struct ttm_bo_device *bdev, struct ttm_mem_reg *mem,
 
 	*virtual = NULL;
 	ret = ttm_mem_io_reserve(bdev, mem);
-	if (ret)
+	if (ret || !mem->bus.is_iomem)
 		return ret;
 
 	if (mem->bus.addr) {