drm/i915: Prevent stalling for a GTT read back from a read-only GPU target

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
index f57536a..af01a58 100644
--- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
+++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
@@ -775,6 +775,7 @@
 		i915_gem_object_move_to_active(obj, ring);
 		if (obj->base.write_domain) {
 			obj->dirty = 1;
+			obj->pending_gpu_write = true;
 			list_move_tail(&obj->gpu_write_list,
 				       &ring->gpu_write_list);
 			intel_mark_busy(ring->dev, obj);