drm/exynos: ipp: add cmd_lock for cmd_list

This patch adds cmd_lock for cmd_list synchronization.

Signed-off-by: YoungJun Cho <yj44.cho@samsung.com>
Acked-by: Seong-Woo Kim <sw0312.kim@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Tested-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>