drm/etnaviv: hold object lock while getting pages for coredump

While all objects that get coredumped have an active IOVA and thus
pages already populated, etnaviv_gem_get_pages() still requires the
object lock to be held.
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>

drivers/gpu/drm/etnaviv/etnaviv_dump.c

@@ -201,7 +201,9 @@ void etnaviv_core_dump(struct etnaviv_gpu *gpu)
 
 		obj = vram->object;
 
+		mutex_lock(&obj->lock);
 		pages = etnaviv_gem_get_pages(obj);
+		mutex_unlock(&obj->lock);
 		if (pages) {
 			int j;