drm/exynos: Fix potential NULL pointer dereference

Pointer was being dereferenced after freeing.

Fixes the following error:
drivers/gpu/drm/exynos/exynos_drm_g2d.c:323 g2d_userptr_put_dma_addr() error:
dereferencing freed memory 'g2d_userptr'

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>