drm/exynos: Fix (more) freeing issues in exynos_drm_drv.c

The following commit [0] fixed a use-after-free, but left the subdrv open
in the error path.

[0] commit 6ca605f7
drm/exynos: Fix freeing issues in exynos_drm_drv.c

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