drm/i915: Pull assert_forcewake_active() underneath the lock

Make no assumption that something in the background is not acquiring the
fw_domain -- but we still do not track owner so assume that any active
domain is intended by the caller.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190707151135.11700-1-chris@chris-wilson.co.uk