drm: virtio: reinstate drm_virtio_set_busid()

Before commit a3257256 ("drm: Lobotomize set_busid nonsense for !pci
drivers"), several DRM drivers for platform devices used to expose an
explicit "drm_driver.set_busid" callback, invariably backed by
drm_platform_set_busid().

Commit a3257256 removed drm_platform_set_busid(), along with the
referring .set_busid field initializations. This was justified because
interchangeable functionality had been implemented in drm_dev_alloc() /
drm_dev_init(), which DRM_IOCTL_SET_VERSION would rely on going forward.

However, commit a3257256 also removed drm_virtio_set_busid(), for
which the same consolidation was not appropriate: this .set_busid callback
had been implemented with drm_pci_set_busid(), and not
drm_platform_set_busid(). The error regressed Xorg/xserver on QEMU's
"virtio-vga" card; the drmGetBusid() function from libdrm would no longer
return stable PCI identifiers like "pci:0000:00:02.0", but rather unstable
platform ones like "virtio0".

Reinstate drm_virtio_set_busid() with judicious use of

  git checkout -p a3257256^ -- drivers/gpu/drm/virtio

Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Joachim Frieben <jfrieben@hotmail.com>
Cc: stable@vger.kernel.org # v4.8
Reported-by: Joachim Frieben <jfrieben@hotmail.com>
Fixes: a3257256
Ref: https://bugzilla.redhat.com/show_bug.cgi?id=1366842


Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>