thunderbolt: Increase runtime PM reference count on DP tunnel discovery

If the driver is unbound and then bound back it goes over the topology
and figure out the existing tunnels. However, if it finds DP tunnel it
should make sure the domain does not runtime suspend as otherwise it
will tear down the DP tunnel unexpectedly.

Fixes: 6ac6faee ("thunderbolt: Add runtime PM for Software CM")
Cc: stable@vger.kernel.org
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>