drm/i915: forcewake warning fixes in debugfs

Some more unsafe debugfs access are fixed with this patch. I tested all reads,
but didn't thoroughly test the writes.

Cc: "Nicolas Kalkhof" <nkalkhof@web.de>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Keith Packard <keithp@keithp.com>