drm/i915: Handle the no-interrupts case for UMS by polling

If the driver calls into the kernel to wait for a breadcrumb to pass,
but hasn't enabled interrupts, fallback to polling the breadcrumb value.

Reported-by: Chris Clayton <chris2553@googlemail.com>
Tested-by: Chris Clayton <chris2553@googlemail.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_irq.c

@@ -1293,12 +1293,12 @@ static int i915_wait_irq(struct drm_device * dev, int irq_nr)
 	if (master_priv->sarea_priv)
 		master_priv->sarea_priv->perf_boxes |= I915_BOX_WAIT;
 
-	ret = -ENODEV;
 	if (ring->irq_get(ring)) {
 		DRM_WAIT_ON(ret, ring->irq_queue, 3 * DRM_HZ,
 			    READ_BREADCRUMB(dev_priv) >= irq_nr);
 		ring->irq_put(ring);
-	}
+	} else if (wait_for(READ_BREADCRUMB(dev_priv) >= irq_nr, 3000))
+		ret = -EBUSY;
 
 	if (ret == -EBUSY) {
 		DRM_ERROR("EBUSY -- rec: %d emitted: %d\n",