drm/mtk: mtk_drm_fb -> drm_framebuffer

Now that mtk_drm_fb is an empty wrapper around drm_framebuffer, we can
just delete it.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Signed-off-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>