drm/exynos: release resources properly when fb creation is failed.

This patch releases allocated resources properly when
exynos_user_fb_create() is failed.

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