drm/exynos: g2d: simplify g2d_free_runqueue_node()

The function is never called with zero 'runqueue_node'.

Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>