drm/i915: debugfs interface for forcewake reference count

forcewake is controlled by the open and close of the debugfs file. This
assures that buggy applications cannot cause the GT to stay on forever.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Keith Packard <keithp@keithp.com>