platform/surface: dtx: Add missing mutex_destroy() call in failure path

When we fail to open the device file due to DTX being shut down, the
mutex is initialized but never destroyed. We are destroying it when
releasing the file, so add the missing call in the failure path as well.

Fixes: 1d609992

 ("platform/surface: Add DTX driver")
Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>
Link: https://lore.kernel.org/r/20210604132540.533036-1-luzmaximilian@gmail.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>