drm/i915: convert force_wake_get to func pointer in the gpu reset code

This was forgotten in the original multi-threaded forcewake
conversion:

commit 8d715f00
Author: Keith Packard <keithp at keithp.com>
Date:   Fri Nov 18 20:39:01 2011 -0800

    drm/i915: add multi-threaded forcewake support

Signed-Off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Keith Packard <keithp@keithp.com>