drm/exynos: Remove explicit freeing using devm_* APIs in exynos_drm_gsc.c

devm_* APIs are device managed and get freed automatically when the
device detaches. Thus explicit freeing is not needed. This saves some
code.

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