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: 5c484cee ("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