Fix type argument for get property call

We were passing the wrong argument in there, it has to be the atom's
type, not the atom property itself.

This fixes detection of the laptop panel in some cases, where we can't
fix it with our fallback mechanism.

Patch-by: Gabriel Souza Franco <gabrielfrancosouza@gmail.com>
Signed-off-by: Sebastian Kügler <sebas@kde.org>