drm/virtio: add virtio-gpu-features debugfs file.

This file prints which features the virtio-gpu device has.

Also add "virtio-gpu-" prefix to the existing fence file,
to make clear this is a driver-specific debugfs file.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190320083612.32067-1-kraxel@redhat.com