drm/exynos: change member variable name.

This patch change current command name from cmd to c_node.
because we are using the member name, 'cmd', for command control ioctl in another structure.
so, this patch changes it to c_node to avoid such confusing.

Signed-off-by: Eunchul Kim <chulspro.kim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>