drm/exynos/fbdev: fix fbdev gem object cleanup

exynos_gem_obj is used by exynos_drm_fbdev_destroy so it cannot be destroyed
before calling the latter. exynos_gem_obj will be destroyed anyway by
exynos_drm_fbdev_destroy->...->exynos_drm_fb_destroy.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>