atomic_get_output_bus_fmts() is only called when the bridge is the last
element in the bridge chain.

If mtk-dpi is not the last bridge, the format of output_bus_cfg is
MEDIA_BUS_FMT_FIXED, and mtk_dpi_dual_edge() will fail to write correct
value to regs.

Fixes: ec8747c5 ("drm/mediatek: dpi: Add bus format negotiation")
Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>