drm: Allow DRM_IOCTL_GET_UNIQUE on render nodes.

On NetBSD, libdrm uses this to discover what kind of bus the device
is on, without which it refuses to expose the render node at all,
rendering it useless.  With this change, libdrm is able to use render
nodes on NetBSD.

Since this is just reading out information about the bus type and
bus/dev/func numbers, I don't think it's problematic to expose to
render nodes.

This requires tweaking the access path to the master.

PR kern/58180

(riastradh)