drm/exynos: Use devm_clk_get in exynos_drm_gsc.c

This eliminates the need for explicit clk_put and makes the
cleanup and exit path code simpler.

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