drm/exynos: fixed bug to exynos_drm_fb_dev_reinit().

this patch solves the problem that fb_helper is released
when exynos_drm_fbdev_reinit() was called. if this function call
is ok then just return.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>