drm/nouveau: 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: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1500837417-40580-33-git-send-email-noralf@tronnes.org

drivers/gpu/drm/nouveau/nouveau_drm.c

@@ -998,7 +998,6 @@ driver_stub = {
 
 	.dumb_create = nouveau_display_dumb_create,
 	.dumb_map_offset = nouveau_display_dumb_map_offset,
-	.dumb_destroy = drm_gem_dumb_destroy,
 
 	.name = DRIVER_NAME,
 	.desc = DRIVER_DESC,