drm: Link directly from drm_master to drm_device

Master-based auth only exists for the legacy/primary drm_minor, hence
there can only be one per device. The goal here is to untangle the
epic dereference games of minor->master and master->minor which is
just massively confusing.

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/1465930269-7883-4-git-send-email-daniel.vetter@ffwll.ch