drm: Nuke SET_UNIQUE ioctl

Ever since

commit 2e1868b560315a8b20d688e646c489a5ad93eeae
Author: Eric Anholt <anholt@freebsd.org>
Date:   Wed Jun 16 09:25:21 2004 +0000

    DRI trunk-20040613 import

the X server supports drm 1.1, thus doesn't call call libdrm's
drmSetBusid - the sole user of this ioctl. When reviewing this note
that for hilarity both the kernel-internal functions (set_busid) and
the libdrm wrapper (drmSetBusid) have names not matching this ioctl
(SET_UNIQUE).

v2: Polish commit message (Emil).

Cc: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466499262-18717-6-git-send-email-daniel.vetter@ffwll.ch