xf86drm: Add drmDevice support for virtio_gpu

The GPU almost exclusively lives on the PCI bus, so we expose it as a
normal PCI one.

This allows all existing drmDevice users to work without any changes.

One could wonder why a separate typeset is not introduced, alike say
host1x. Unlike host1x the PCI/platform distinction for virtio provides
no extra information. Plus if needed we can add the separate set at a
later stage.

Here are a few 'features' that virtio seems to be missing:
 - provides extra information on top the plaform devices
 - supports a range of GPU devices
 - is considered hardware description (DT)

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Tested-by: Robert Foss <robert.foss@collabora.com>
Reviewed-by: Robert Foss <robert.foss@collabora.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>