drm/i915: Explicitly track active fw_domain timers

Stop guessing over whether we have an extra wakeref held by the delayed
fw put, and track it explicitly for the sake of debug.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190708154914.26850-1-chris@chris-wilson.co.uk