drm: export drm_vm_open_locked

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: Arnd 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: Dave Airlie <airlied@redhat.com>