drm: fix missing reference counting decrease

commit dabe1954 upstream.

In drm_dp_mst_allocate_vcpi, it returns true in two paths,
but in one path, there is no reference couting decrease.

Signed-off-by: Insu Yun <wuninsu@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>