drm: call drm_unplug_minor() from drm_put_minor()

This protects drm_unplug_minor() against repeated calls so we can use it
in drm_put_minor(). This allows us to further simplify it in follow-ups as
we no longer do minor-destruction in both functions but only in
drm_unplug_minor().
Also add kernel-doc comments about what these calls do.

[airlied: fixup for changes to kdev stuff]
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>