drm/virtio: allocate some extra bufs

virtio-gpu guest driver appearently can run out of buffers.
allocate some extra buffers, as quick stopgap for 4.9.
analyzing root cause and fixing it properly is TBD.

Reported-by: Jiri Slaby <jslaby@suse.cz>
Tested-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>