drm/exynos: Remove fb_helper from struct exynos_drm_private

The DRM device stores a pointer to the fbdev helper. Remove struct
exynos_drm_private.fb_helper, which contains the same value. 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>