drm/i915: Only emit the 'send bug report' once for a GPU hang

Use a locked xchg to ensure that the global log message giving
instructions on how to send a bug report is emitted precisely once.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190819075835.20065-5-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gpu_error.c

@@ -1764,7 +1764,7 @@ void i915_capture_error_state(struct drm_i915_private *i915,
 		return;
 	}
 
-	if (!warned &&
+	if (!xchg(&warned, true) &&
 	    ktime_get_real_seconds() - DRIVER_TIMESTAMP < DAY_AS_SECONDS(180)) {
 		pr_info("GPU hangs can indicate a bug anywhere in the entire gfx stack, including userspace.\n");
 		pr_info("Please file a _new_ bug report on bugs.freedesktop.org against DRI -> DRM/Intel\n");
@@ -1772,7 +1772,6 @@ void i915_capture_error_state(struct drm_i915_private *i915,
 		pr_info("The GPU crash dump is required to analyze GPU hangs, so please always attach it.\n");
 		pr_info("GPU crash dump saved to /sys/class/drm/card%d/error\n",
 			i915->drm.primary->index);
-		warned = true;
 	}
 }