drm: Make locked tasklet handling more robust.

Initialize the spinlock unconditionally when struct drm_device is filled in,
and return early in drm_locked_tasklet() if the driver doesn't support IRQs.

Signed-off-by: Dave Airlie <airlied@linux.ie>
diff --git a/drivers/char/drm/drm_stub.c b/drivers/char/drm/drm_stub.c
index 6f748e1..5fd6dc0 100644
--- a/drivers/char/drm/drm_stub.c
+++ b/drivers/char/drm/drm_stub.c
@@ -61,6 +61,7 @@
 
 	spin_lock_init(&dev->count_lock);
 	spin_lock_init(&dev->drw_lock);
+	spin_lock_init(&dev->tasklet_lock);
 	init_timer(&dev->timer);
 	mutex_init(&dev->struct_mutex);
 	mutex_init(&dev->ctxlist_mutex);