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>