drm/exynos: fix not to remain exynos_gem_obj as a leak

The exynos_drm_gem_create() only calls drm_gem_object_release()
when exynos_drm_alloc_buf() is failed, and exynos_gem_obj remains
as a leak, which is allocated in exynos_drm_gem_init().
So this patch fixes it not to remain as a leak.

Signed-off-by: YoungJun Cho <yj44.cho@samsung.com>
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>