drm: use dev_name as default unique name in drm_dev_alloc()

The following code pattern exists in some DRM drivers:

    ddev = drm_dev_alloc(&driver, parent_dev);
    drm_dev_set_unique(ddev, dev_name(parent_dev));

(Sometimes dev_name(ddev->dev) is used, which is the same.)

As suggested in
http://lists.freedesktop.org/archives/dri-devel/2015-December/096441.html,
the unique name of a new DRM device can be set as dev_name(parent_dev)
when parent_dev is not NULL (vgem is a special case).

Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>