drm/vmwgfx: fix a warning message

The WARN_ON() macro only takes a condition argument, it doesn't take
a message.  I have converted this to WARN() instead.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>

drivers/gpu/drm/vmwgfx/vmwgfx_fifo.c

@@ -390,7 +390,7 @@ void *vmw_fifo_reserve_dx(struct vmw_private *dev_priv, uint32_t bytes,
 	else if (ctx_id == SVGA3D_INVALID_ID)
 		ret = vmw_local_fifo_reserve(dev_priv, bytes);
 	else {
-		WARN_ON("Command buffer has not been allocated.\n");
+		WARN(1, "Command buffer has not been allocated.\n");
 		ret = NULL;
 	}
 	if (IS_ERR_OR_NULL(ret)) {