drm/exynos: Remove redundant NULL check in exynos_drm_gsc.c

devm_request_and_ioremap API checks for NULL. Hence explicit
NULL check is not necessary. Saves some code.

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