drm/exynos: Initialize fbdev DRM client

Initialize the fbdev client in the fbdev code with empty helper
functions. Also clean up the client. The helpers will later
implement various functionality of the DRM client. No functional
changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>