drm/i915: Double check bumping after the spinlock

In preparation for making GEM execbuf parallel, we need to be prepared
to handle very early declaration of dependencies -- even before our
signaler has itself been submitted.

References: a79ca656b6 ("drm/i915: Push the wakeref->count deferral to the backend")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200220123608.1666271-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_scheduler.c

@@ -363,6 +363,9 @@ static void __bump_priority(struct i915_sched_node *node, unsigned int bump)
 {
 	struct i915_sched_attr attr = node->attr;
 
+	if (attr.priority & bump)
+		return;
+
 	attr.priority |= bump;
 	__i915_schedule(node, &attr);
 }