drivers: gpu: Remove unused function in ttm_lock.c

Remove unused function ttm_write_lock_downgrade() from
drm/ttm/ttm_lock.c.

This eliminates the following warning in drm/ttm/ttm_lock.c:
drivers/gpu/drm/ttm/ttm_lock.c:189:6: warning: no previous prototype for ‘ttm_write_lock_downgrade’ [-Wmissing-prototypes]
Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>

drivers/gpu/drm/ttm/ttm_lock.c

@@ -186,14 +186,6 @@ int ttm_write_lock(struct ttm_lock *lock, bool interruptible)
 }
 EXPORT_SYMBOL(ttm_write_lock);
 
-void ttm_write_lock_downgrade(struct ttm_lock *lock)
-{
-	spin_lock(&lock->lock);
-	lock->rw = 1;
-	wake_up_all(&lock->queue);
-	spin_unlock(&lock->lock);
-}
-
 static int __ttm_vt_unlock(struct ttm_lock *lock)
 {
 	int ret = 0;