drm/exynos: add error check routine in exynos_drm_open

When the exynos_drm_subdrv_open() returns error, the file_priv
should be released and file->driver_priv set to NULL.

Signed-off-by: YoungJun Cho <yj44.cho@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>