drm/mediatek: Add missing put_device() call in mtk_drm_kms_init()

if of_find_device_by_node() succeed, mtk_drm_kms_init() doesn't have
a corresponding put_device(). Thus add jump target to fix the exception
handling for this function implementation.

Fixes: 119f5173

 ("drm/mediatek: Add DRM Driver for Mediatek SoC MT8173.")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>