drm/exynos: g2d: fix null pointer dereference

This patch fixes a null pointer dereference issue incurred by
calling g2d_remove when exynos_drm_platform_probe is failed.

cmdlist_pool of g2d is allocated when g2d sub driver is probed.
So if exynos_drm_platform_probe is failed, the g2d sub driver is
not probed and the cmdlist_pool is still NULL.

Signed-off-by: Inki Dae <inki.dae@samsung.com>