damage: Implicitly unregister on destroy

There's no reason not to, and it simplifies quite a few callers.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Adam Jackson <ajax@redhat.com>

exa/exa_classic.c

@@ -192,7 +192,6 @@ exaModifyPixmapHeader_classic(PixmapPtr pPixmap, int width, int height,
          * gpu memory, so there's no need to track damage.
          */
         if (pExaPixmap->pDamage) {
-            DamageUnregister(&pPixmap->drawable, pExaPixmap->pDamage);
             DamageDestroy(pExaPixmap->pDamage);
             pExaPixmap->pDamage = NULL;
         }