drm/virtio: make mmap callback consistent with callbacks

This is a very, very minor cleanup.

Signed-off-by: Gurchetan Singh <gurchetansingh@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20200227002601.745-2-gurchetansingh@chromium.org


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>