drm/virtio: add missing virtio_gpu_array_lock_resv call

When submitting a fenced command we must lock the object reservations
because virtio_gpu_queue_fenced_ctrl_buffer() unlocks after adding the
fence.

Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Tested-by: Jann Horn <jannh@google.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200110094535.23472-1-kraxel@redhat.com