drm/i915: Fix build without CONFIG_PM

drivers/gpu/drm/i915/i915_debugfs.c: In function ‘i915_runtime_pm_status’:
drivers/gpu/drm/i915/i915_debugfs.c:2528:34: error: ‘struct dev_pm_info’ has no member named ‘usage_count’
      atomic_read(&dev->dev->power.usage_count));

Regression from commit a6aaec8b
Author: Damien Lespiau <damien.lespiau@intel.com>
Date:   Thu Jun 4 18:23:58 2015 +0100

    drm/i915: Add runtime PM's usage_count in i915_runtime_pm_status
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_debugfs.c

@@ -2490,8 +2490,12 @@ static int i915_runtime_pm_status(struct seq_file *m, void *unused)
 	seq_printf(m, "GPU idle: %s\n", yesno(!dev_priv->mm.busy));
 	seq_printf(m, "IRQs disabled: %s\n",
 		   yesno(!intel_irqs_enabled(dev_priv)));
+#ifdef CONFIG_PM
 	seq_printf(m, "Usage count: %d\n",
 		   atomic_read(&dev->dev->power.usage_count));
+#else
+	seq_printf(m, "Device Power Management (CONFIG_PM) disabled\n");
+#endif
 
 	return 0;
 }