drm/ast: 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>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1502034068-51384-11-git-send-email-noralf@tronnes.org

drivers/gpu/drm/ast/ast_drv.c

@@ -209,7 +209,6 @@ static struct drm_driver driver = {
 	.gem_free_object_unlocked = ast_gem_free_object,
 	.dumb_create = ast_dumb_create,
 	.dumb_map_offset = ast_dumb_mmap_offset,
-	.dumb_destroy = drm_gem_dumb_destroy,
 
 };