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: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>