drm/virtio: fix virtio_gpu_cursor_plane_update().

Add missing virtio_gpu_array_lock_resv() call.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200211135047.22261-3-kraxel@redhat.com