drm: export drm_vm_open_locked

The EXYNOS DRM driver uses drm_vm_open_locked in its mmap() function,
and it can be built as a loadable module, which currently fails.
This exports the symbol from the DRM core to avoid

ERROR: "drm_vm_open_locked" [drivers/gpu/drm/exynos/exynosdrm.ko] undefined!
Signed-off-by: NArnd Bergmann <arnd@arndb.de>
Cc: Inki Dae <inki.dae@samsung.com>
Cc: Joonyoung Shim <jy0922.shim@samsung.com>
Cc: Seung-Woo Kim <sw0312.kim@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: NDave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_vm.c

@@ -422,6 +422,7 @@ void drm_vm_open_locked(struct drm_device *dev,
 		list_add(&vma_entry->head, &dev->vmalist);
 	}
 }
+EXPORT_SYMBOL_GPL(drm_vm_open_locked);
 
 static void drm_vm_open(struct vm_area_struct *vma)
 {