drm/sun4i: add missing put_device() call in sun8i_r40_tcon_tv_set_mux()

If sun8i_r40_tcon_tv_set_mux() succeed, sun8i_r40_tcon_tv_set_mux()
doesn't have a corresponding put_device(). Thus add put_device()
to fix the exception handling for this function implementation.

Fixes: 0305189a ("drm/sun4i: tcon: Add support for R40 TCON")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20200826010826.1785487-1-yukuai3@huawei.com