drivers: gpu: Mark function as static in sis_drv.c

Mark function as static because it is not used outside the file
drm/sis/sis_drv.c.

This eliminates the following warning in drm/sis/sis_drv.c:
drivers/gpu/drm/sis/sis_drv.c:97:6: warning: no previous prototype for ‘sis_driver_postclose’ [-Wmissing-prototypes]
Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/sis/sis_drv.c

@@ -94,7 +94,7 @@ static int sis_driver_open(struct drm_device *dev, struct drm_file *file)
 	return 0;
 }
 
-void sis_driver_postclose(struct drm_device *dev, struct drm_file *file)
+static void sis_driver_postclose(struct drm_device *dev, struct drm_file *file)
 {
 	struct sis_file_private *file_priv = file->driver_priv;