drm/i915: Complain if hsw_get_pipe_config acquires the same power well twice

As we only release each power well once, we assume that each transcoder
maps to a different domain. Complain if this is not so.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190114142129.24398-19-chris@chris-wilson.co.uk