drm/exynos/ipp: replace work_struct casting with better constructs

Type casting should be avoided if possible. In case of
work_struct it can be simply replaced by reference to member field.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>