drm/exynos: gsc: prepare and unprepare gsc clock

Ths patch changes the clk_enable and clk_disable call in gsc driver
into clk_prepare_enable and clk_disable_unprepare.

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>