drm/exynos: Use devm_kzalloc in exynos_drm_ipp.c

devm_kzalloc makes the code simpler by eliminating the need for
explicit freeing.

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