drm/exynos: Keep a reference to frame buffer GEM objects

GEM objects used by frame buffers must be referenced for the whole life
of the frame buffer. Release the references in the frame buffer
destructor instead of its constructor.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>