drm/i915: catch forcewake reference underruns

Without this the new drv_suspend/forcewake subtest I've created
doesn't result in immediately visible failures.

Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Ben Widawsky <ben@bwidawsk.net>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>