drm/exynos: exynos_drm_ipp: Fix incorrect usage of IS_ERR_OR_NULL

None of these functions actually return a NULL pointer. Hence use
IS_ERR() instead.

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