drm/exynos: fixed wrong err ptr usage and destroy call in exeception

- exynos_drm_buf_create() returns err pointer so NULL check is wrong.
- Case that exynos_gem_obj is not created, destroy call in exception
  handle lable uses this pointer. so instead buffer is directly used.

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>