UPSTREAM: 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


(cherry picked from commit 6bbeae35b67b339b81e42c9ab56789788491dc00)
Signed-off-by: Greg Hartman <ghartman@google.com>
BUG: 139386237
Change-Id: I35c5b4baadc240bbe0a452a1ee265be1dc2cd53c