drm/ttm/tests: Fix build failure on PREEMPT_RT

Fix a compile error in the kunit tests when CONFIG_PREEMPT_RT is
enabled, and the normal mutex is converted into a rtmutex.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202602261547.3bM6yVAS-lkp@intel.com/
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patch.msgid.link/20260304085616.1216961-1-dev@lankhorst.se
Signed-off-by: Maarten Lankhorst <dev@lankhorst.se>

Author: Maarten Lankhorst

drivers/gpu/drm/ttm/tests/ttm_bo_test.c

@@ -222,13 +222,13 @@ static void ttm_bo_reserve_interrupted(struct kunit *test)
 		KUNIT_FAIL(test, "Couldn't create ttm bo reserve task\n");
 
 	/* Take a lock so the threaded reserve has to wait */
-	mutex_lock(&bo->base.resv->lock.base);
+	dma_resv_lock(bo->base.resv, NULL);
 
 	wake_up_process(task);
 	msleep(20);
 	err = kthread_stop(task);
 
-	mutex_unlock(&bo->base.resv->lock.base);
+	dma_resv_unlock(bo->base.resv);
 
 	KUNIT_ASSERT_EQ(test, err, -ERESTARTSYS);
 }