drm/exynos: Use devm_* functions in exynos_drm_g2d.c file

devm_* functions are device managed functions and make error handling
and cleanup cleaner and simpler.

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