drm/vkms: Release pages_lock before return

Release pages_lock before return when vkms_obj->vaddr is NULL.
This patch fixes: 6c234fe3 ("drm/vkms: Implement CRC debugfs API").

Signed-off-by: Haneen Mohammed <hamohammed.sa@gmail.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180803201142.GA2206@haneenDRM