drm/i915: Fix wakeref cookie handling in debugfs/i915_forcewake_user

To avoid a false positive of a leaked wakeref, we can store the cookie
in file->private_data and use it in intel_runtime_pm_put.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190117144831.13156-1-tvrtko.ursulin@linux.intel.com