drm/i915: save/resume forcewake lock fixes

The lock must be held for the saving and restoring of VGA state.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
CC: Alexander Zhaunerchyk <alex.vizor@gmail.com>
CC: Andrey Rahmatullin <wrar@wrar.name>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Keith Packard <keithp@keithp.com>