drm/exynos: Make ipp_handle_cmd_work static

Fixes the following warning:
drivers/gpu/drm/exynos/exynos_drm_ipp.c:872:6: warning:
symbol 'ipp_handle_cmd_work' was not declared. Should it be static?
Signed-off-by: NSachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: NInki Dae <inki.dae@samsung.com>

drivers/gpu/drm/exynos/exynos_drm_ipp.c

@@ -869,7 +869,7 @@ static void ipp_put_event(struct drm_exynos_ipp_cmd_node *c_node,
 	}
 }
 
-void ipp_handle_cmd_work(struct device *dev,
+static void ipp_handle_cmd_work(struct device *dev,
 		struct exynos_drm_ippdrv *ippdrv,
 		struct drm_exynos_ipp_cmd_work *cmd_work,
 		struct drm_exynos_ipp_cmd_node *c_node)