drm/mediatek: dp: Reduce indentation in mtk_dp_bdg_detect()

In order to improve human readability, reduce the indentation by
returning early if the dp/edp cable is not plugged in.

Signed-off-by: Bo-Chen Chen <rex-bc.chen@mediatek.com>
Acked-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220916133821.27980-2-rex-bc.chen@mediatek.com