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_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 590d8f2..7b37c19 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -729,6 +729,12 @@
 	unsigned int dirty : 1;
 
 	/**
+	 * This is set if the object has been written to since the last
+	 * GPU flush.
+	 */
+	unsigned int pending_gpu_write : 1;
+
+	/**
 	 * Fence register bits (if any) for this object.  Will be set
 	 * as needed when mapped into the GTT.
 	 * Protected by dev->struct_mutex.