drm/qxl: Remove format string errors

Enable format string checks for qxl_io_log and remove resulting warnings
which could lead to memory errors on different platform or just printing
wrong information.

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/qxl/qxl_drv.h b/drivers/gpu/drm/qxl/qxl_drv.h
index 7c6cafe..d854969 100644
--- a/drivers/gpu/drm/qxl/qxl_drv.h
+++ b/drivers/gpu/drm/qxl/qxl_drv.h
@@ -328,7 +328,7 @@
 };
 
 /* forward declaration for QXL_INFO_IO */
-void qxl_io_log(struct qxl_device *qdev, const char *fmt, ...);
+__printf(2,3) void qxl_io_log(struct qxl_device *qdev, const char *fmt, ...);
 
 extern const struct drm_ioctl_desc qxl_ioctls[];
 extern int qxl_max_ioctl;