drm/exynos: clear node object type at gem unmap

This patch clears node object type in G2D unmap cmdlist.
The obj_type of cmdlist node has to be cleared in
g2d_unmap_cmdlist_gem() so that the node can be reused
in g2d_map_cmdlist_gem().

Signed-off-by: YoungJun Cho <yj44.cho@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>