drm/virtio: add edid support

linux guest driver implementation of the VIRTIO_GPU_F_EDID feature.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20181030063206.19528-3-kraxel@redhat.com