drm/exynos: exynos_drm_fb -> drm_framebuffer

Now exynos_drm_fb is just an empty wrapper around drm_framebuffer, we
can drop it.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Cc: Inki Dae <inki.dae@samsung.com>
Cc: Joonyoung Shim <jy0922.shim@samsung.com>
Cc: Seung-Woo Kim <sw0312.kim@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>