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>