drm: fix missing reference counting decrease

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>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>