drm/i915: forcewake struct mutex locking fixes

Found by the new strict checking for the mutex being held whilst
manipulating the forcewake status.

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>