drm: export drm_global_mutex for drivers to use

Nouveau needs to be able to drop the mutex before sleeping to prevent a
deadlock from occuring.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/drm_fops.c

@@ -41,6 +41,7 @@
 
 /* from BKL pushdown: note that nothing else serializes idr_find() */
 DEFINE_MUTEX(drm_global_mutex);
+EXPORT_SYMBOL(drm_global_mutex);
 
 static int drm_open_helper(struct inode *inode, struct file *filp,
 			   struct drm_device * dev);