We added this WARN_ON to guard against using uninitialized
forcewake domains. But forgot blissfully that not all
gens have forcewake domains in the first place.

v2: Move WARN_ON to fw_domains_init (Chris)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=88911
Tested-by: Ding Heng <hengx.ding@intel.com> (v1)
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[Jani: add comment above WARN_ON as suggested by Chris]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>