drm: Move authmagic cleanup into drm_master_release

It's related, and soon authmagic will also use the master_mutex.

There is an ever-so-slightly semantic change here:
- authmagic will only be cleaned up for primary_client drm_minors. But
  it's impossible to create authmagic on render/control nodes, so this
  is fine.
- The cleanup is moved down a bit in the release processing. Doesn't
  matter at all since authmagic is purely internal logic used by the
  core ioctl access checks, and when we're in a file's release
  callback no one can do ioctls any more.

v2: Rebased.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> (v1)
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/1466148814-8194-3-git-send-email-daniel.vetter@ffwll.ch