drm/exynos: Remove struct exynos_drm_fbdev

Remove struct exynos_drm_fbdev, which is an empty wrapper around
struct drm_fb_helper. Use the latter directly. 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>