drm/vmwgfx: Fix a potential infinite spin waiting for fifo idle

commit f01ea0c3 upstream.

The code waiting for fifo idle was incorrect and could possibly spin
forever under certain circumstances.
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reported-by: Mark Sheldon <markshel@vmware.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>
Reivewed-by: Mark Sheldon <markshel@vmware.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/gpu/drm/vmwgfx/vmwgfx_fifo.c

@@ -163,8 +163,9 @@ void vmw_fifo_release(struct vmw_private *dev_priv, struct vmw_fifo_state *fifo)
 
 	mutex_lock(&dev_priv->hw_mutex);
 
-	while (vmw_read(dev_priv, SVGA_REG_BUSY) != 0)
 	vmw_write(dev_priv, SVGA_REG_SYNC, SVGA_SYNC_GENERIC);
+	while (vmw_read(dev_priv, SVGA_REG_BUSY) != 0)
+		;
 
 	dev_priv->last_read_seqno = ioread32(fifo_mem + SVGA_FIFO_FENCE);