drm/virtio: add virtio_gpu_object_array & helpers

Some helper functions to manage an array of gem objects.

v9: use dma_resv_lock_interruptible.
v6:
 - add ticket to struct virtio_gpu_object_array.
 - add virtio_gpu_array_{lock,unlock}_resv helpers.
 - add virtio_gpu_array_add_fence helper.
v5: some small optimizations (Chia-I Wu).
v4: make them virtio-private instead of generic helpers.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190829103301.3539-8-kraxel@redhat.com