drm: Check for drm_device->dev in drm_set_busid

I've failed to remember that we have virtual drivers like vgem which
have no underlying struct device. Fix this asap.

Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Fixes: 5c484cee7ef9 ("drm: Remove drm_driver->set_busid hook")
Cc: Thierry Reding <treding@nvidia.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170621130429.20537-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_ioctl.c

@@ -143,7 +143,7 @@ static int drm_set_busid(struct drm_device *dev, struct drm_file *file_priv)
 	if (master->unique != NULL)
 		drm_unset_busid(dev, master);
 
-	if (dev_is_pci(dev->dev)) {
+	if (dev->dev && dev_is_pci(dev->dev)) {
 		ret = drm_pci_set_busid(dev, master);
 		if (ret) {
 			drm_unset_busid(dev, master);