drm/cirrus: Use the drm_driver.dumb_destroy default

drm_gem_dumb_destroy() is the drm_driver.dumb_destroy default,
so no need to set it.

Cc: Dave Airlie <airlied@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1502034068-51384-8-git-send-email-noralf@tronnes.org

drivers/gpu/drm/cirrus/cirrus_drv.c

@@ -142,7 +142,6 @@ static struct drm_driver driver = {
 	.gem_free_object_unlocked = cirrus_gem_free_object,
 	.dumb_create = cirrus_dumb_create,
 	.dumb_map_offset = cirrus_dumb_mmap_offset,
-	.dumb_destroy = drm_gem_dumb_destroy,
 };
 
 static const struct dev_pm_ops cirrus_pm_ops = {