anv: Set correct write domain on window system BOs

We need to make sure GEM understands that we're writing to the BO, in
case it needs to synchronize with other rings (blitter use in display
server, for example).
diff --git a/src/vulkan/anv_device.c b/src/vulkan/anv_device.c
index dfc29e4..c3a370b 100644
--- a/src/vulkan/anv_device.c
+++ b/src/vulkan/anv_device.c
@@ -1070,6 +1070,7 @@
    bo->index = 0;
    bo->offset = 0;
    bo->size = size;
+   bo->is_winsys_bo = false;
 
    return VK_SUCCESS;
 }