drm: Protect master->unique with dev->master_mutex

No one looks at the major/minor versions except the unique/busid
stuff. If we protect that with the master_mutex (since it also affects
the unique of each master, oh well) we can mark these two IOCTL with
DRM_UNLOCKED.

While doing this I realized that the comment for the magic_map is
outdated, I've forgotten to update it in:

commit d2b34ee6
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Jun 17 09:33:21 2016 +0200

    drm: Protect authmagic with master_mutex

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
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/20161210215255.7765-1-daniel.vetter@ffwll.ch