drm/exynos: Fix freeing issues in exynos_drm_drv.c

Fixes the following errors:
drivers/gpu/drm/exynos/exynos_drm_drv.c:182 exynos_drm_open()
error: double free of 'file_priv'
drivers/gpu/drm/exynos/exynos_drm_drv.c:188 exynos_drm_open()
error: dereferencing freed memory 'file_priv'

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