backend-drm: Do not pass a NULL reason for trying out scanout
We shouldn't be assuming that we can always import the framebuffer, so don't pass a NULL, which can later be used to add a failure reason. If we can't test a scanout directly, we might end up with a valid reason.
Seen when using direct-display proto extension and failing when testing out scanout.
Signed-off-by: Marius Vlad marius.vlad@collabora.com