drm: Push struct_mutex into ->master_destroy

Only two drivers implement this hook. vmwgfx (which doesn't need it
really) and legacy radeon (which since v1 has been nuked, yay).

v1: Rebase over radeon ums removal.

Cc: Thomas Hellstrom <thellstrom@vmware.com>
Cc: Alex Deucher <alexdeucher@gmail.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NAlex Deucher <alexander.deucher@amd.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1461691808-12414-6-git-send-email-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_drv.c

@@ -123,10 +123,10 @@ static void drm_master_destroy(struct kref *kref)
 	struct drm_device *dev = master->minor->dev;
 	struct drm_map_list *r_list, *list_temp;
 
-	mutex_lock(&dev->struct_mutex);
 	if (dev->driver->master_destroy)
 		dev->driver->master_destroy(dev, master);
 
+	mutex_lock(&dev->struct_mutex);
 	list_for_each_entry_safe(r_list, list_temp, &dev->maplist, head) {
 		if (r_list->master == master) {
 			drm_legacy_rmmap_locked(dev, r_list->map);