thunderbolt: Add KUnit test for devices with no DisplayPort adapters

Add a KUnit test to check that buffer allocation works also for devices
with no DP adapters.

Signed-off-by: Gil Fine <gil.fine@intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
(cherry picked from commit c7c99a09)