drm/rockchip: rockchip_drm_fb -> drm_framebuffer

Now that rockchip_drm_fb is just a wrapper around drm_framebuffer, we
can remove it.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Acked-by: Heiko Stübner <heiko@sntech.de>
Tested-by: Heiko Stübner <heiko@sntech.de>
Cc: Sandy Huang <hjc@rock-chips.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180330141138.28987-5-daniels@collabora.com