sna: Correct assertion for a partial read buffer

The batch may legitimately be submitted prior to the attachment of the
read buffer, if, for example, we need to switch rings. Therefore update
the assertion to only check that the bo remains in existence via either
a reference from the exec or from the user

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/sna/kgem.c

@@ -1224,7 +1224,7 @@ static void kgem_finish_partials(struct kgem *kgem)
 
 	list_for_each_entry_safe(bo, next, &kgem->partial, base.list) {
 		if (!bo->write) {
-			assert(bo->base.exec);
+			assert(bo->base.exec || bo->base.refcnt > 1);
 			goto decouple;
 		}