drm/mtk: Remove impossible internal error

We cannot create a framebuffer with no objects, so there's no point
testing for it.

v2: Remove the error entirely. (Sean, CK, Thierry)

Signed-off-by: Daniel Stone <daniels@collabora.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Thierry Reding <treding@nvidia.com>
Cc: CK Hu <ck.hu@mediatek.com>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>