drm/virtio: fix a file name comment reference

Easier to find where declarations are implemented.

Signed-off-by: Anthoine Bourgeois <anthoine.bourgeois@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20201119010809.528-2-gurchetansingh@chromium.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>