drm/exynos/ipp: remove struct exynos_drm_ipp_private

struct exynos_drm_ipp_private contains only one pointer so all occurrences
of the struct can be replaced by the pointer itself.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>