drm/exynos: consider common clock framework to g2d driver.

This patch just changes clk_enable/disable to
clk_prepare_enable/clk_disable_unprepare, and
adds related exception codes.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>