drm/exynos: Stop using drm_framebuffer_unregister_private

This is the deprecated function for when you embedded the framebuffer
somewhere else (which breaks refcounting). But exynos is using
drm_framebuffer_remove and a free-standing fb, so this is rendundant.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>