drm/virtio: bump driver version after explicit synchronization addition

To reflect the (backward compatible) changes in the uabi we are bumping
the driver's version.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co>
Signed-off-by: Robert Foss <robert.foss@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20181112165157.32765-5-robert.foss@collabora.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>