drm/virtio: Remove return from void function

This patch fixes the checkpatch.pl warning:

virtgpu_ttm.c:181: WARNING: void function return statements are not
generally useful
...

Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/fd8dc6599c81c7aec6753c8552c1cabb7baa7577.1519343668.git.rodrigosiqueiramelo@gmail.com


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