drm/mtk: Move GEM BO to drm_framebuffer

Since drm_framebuffer can now store GEM objects directly, place them
there rather than in our own subclass. As this makes the framebuffer
create_handle and destroy functions the same as the GEM framebuffer
helper, we can reuse those.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20180518134705.12533-2-daniels@collabora.com